Nelson Mandela remains a 20th century icon as a champion of human rights, political activism, equality and peace.

On what would have been his 100th birthday today, people across the world are celebrating his legacy and calling for leaders to do more to tackle global injustice and poverty.

In 1962, Mandela was arrested for his activism against South Africa’s racist apartheid system.

FEBRUARY 13: Nelson Mandela gestured to supporters in Soweto two days after his release from prison in Cape Town. He addressed more than 100,000 people inside a soccer stadium saying, "during the past 27 years I have looked forward to this day when I would come back to the area I regard as home, to meet my brothers and sisters and grandchildren." (Photo by Joanne Rathe/The Boston Globe via Getty Images)
Nelson Mandela spoke to more than 100,000 supporters in Soweto two days after his release from prison in Cape Town saying ‘during the past 27 years I have looked forward to this day when I would come back to the area I regard as home, to meet my brothers and sisters and grandchildren’ (Picture: Getty)

He was sentenced to life imprisonment for conspiring to overthrow the apartheid state and served 27 years behind bars, split between Robben Island, Pollsmoor Prison, and Victor Verster Prison.

Amid growing domestic and international pressure – as well as increased fears of a racial civil war – then-president FW de Klerk released him in 1990.

Three years after his release, he became the first black president of South Africa and went on to win the Nobel Peace Prize.

Five years after his death in 2013, he continues to provide inspiration in the face of adversity and injustice.

ANC president Nelson Mandela is surrounded by young supporters after addressing residents at Phola Park, a squatter settlement east of Johannesburg, 31 May 1992. In his speech, he attacked President Frederik W. de Klerk of being responsible for the violence that killed scores of people in black townships. (Photo credit should read WALTER DHLADHLA/AFP/Getty Images)
ANC president Nelson Mandela surrounded by young supporters after addressing residents at Phola Park, a squatter settlement east of Johannesburg, in May 1992 (Picture: Getty)

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BOSTON - JUNE 23: Nelson Mandelas visit to Boston in 1990 JULY 17, 2013 ARTICLE PHOTOS COMMENTS As the antiapartheid leader Nelson Mandela turns 95, we look at his first trip to Boston in 1990. This was part of his first visit to the United States since he was freed after 27 years as a political prisoner in South Africa. Throngs of people gathered in Boston to see their hero and celebrate his movement during his two-day stay. - Leanne Burden Seidel and Lisa Tuite JOHN TLUMACKI/GLOBE STAFF June 23, 1990: Spectators greeted the limousine carrying Nelson Mandela on Martin Luther King Boulevard. During a whirlwind tour through Roxbury and Dorchester, the Mandelas were welcomed as heroes. Thousands lined the streets and waved the African National Congress's black, green, and gold flag. Others placed pictures of the Mandelas in their windows. RELATED: FULL SCREEN JONATHAN WIGGS/GLOBE STAFF June 23, 1990: Nelson Mandela acknowledged the cheers after his speech at the Hatch Shell. His then-wife Winnie is at right. His remarks praised Boston and Massachusetts for their fight against apartheid. "It is with boundless joy that we speak to you today. We embrace each and every one of you. Sisters and brothers, my delegation, my wife and I are deeply moved by the warmth and love that your city has accorded us. We are even more touched that it was here in Boston that your own independence movement got its birth. Your love and enthusiasm as well as your pioneering role in taking positive measures against apartheid portray a deep feeling of kinship you hold towards our people and the just cause that they are so vigorously fighting for. When one day our history is rewritten, the pioneering and leading role of Massachusetts will stand out like a shining diamond. It was you who supported us when very few knew of our existence, our trial and tribulation." (Photo by Jonathan Wiggs/The Boston Globe via Getty Images)
Mandelas visit to Boston, his first to the US after being released from prison in 1990 (Picture: Getty)
Former South African President Nelson Mandela (L) claps his hands after American pop-icon Michael Jackson was given a lifetime achievement award during the Kora All Africa music awards in Sun City some 120 km northeast of Johannesburg , 04 September 1999. (Photo credit should read ADIL BRADLOW/AFP/Getty Images)
Mandela claps his hands after American pop-icon Michael Jackson was given a lifetime achievement award during the Kora All Africa music awards in Johannesburg in 1999. (Picture: AFP/Getty)
GLASGOW, UNITED KINGDOM - OCTOBER 9: ANC President Nelson Mandela (R) and an unidentified singer dance for freedom at a rally upon his arrival to Glasgow, Scotland, 09 October. (Photo credit should read PA/AFP/Getty Images)
ANC President Nelson Mandela and an unidentified singer dance for freedom at a rally upon his arrival to Glasgow, Scotland in 2009 (Picture: PA/AFP/Getty)
Britain's Queen Elizabeth II (left) meets with former South African President, Nelson Mandela during a reception at Buckingham Palace, London, 20 October 2003, to mark the centenary of the Rhodes Trust, started by the 19th century entrepreneur, Cecil John Rhodes, which provides scholarships for students around the world to study at Oxford University. The former President of South Africa has entered into partnership with the Rhodes Trust, in its centenary year, to establish the Mandela Rhodes Foundation. At present, 93 scholars are selected from 24 countries each year for study at Oxford University. (Photo credit should read KIRSTY WIGGLESWORTH/AFP/Getty Images)
Mandela met the Queen at Buckingham Palace in 2003, to mark the centenary of the Rhodes Trust which provides scholarships for students around the world to study at Oxford University (Picture: AFP/Getty)

Paying tribute in an impassioned speech to mark the 100th anniversary, former US president Barack Obama told a cheering crowd of thousands in South Africa to keep alive the ideas that Mandela worked for including democracy, diversity and tolerance.

He said: ‘I believe in Nelson Mandela’s vision. I believe in a vision shared by Gandhi and [Martin Luther] King and Abraham Lincoln.

‘I believe in a vision… built on the premise that all people are created equal and they’re endowed by our creator with certain inalienable rights.’

The Duke and Duchess of Sussex also paid tribute to the former South African leader in a visit to an exhibition about Mandela’s life in London.

Harry and Meghan’s attention was well received by those who knew Mandela well.

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Video: Barack Obama delivers remarks at Nelson Mandela lecture

The former US president appeared at the 16th Nelson Mandela Lecture in Johannesburg, where without naming names he criticised strongman politics and politicians willingness to lie after being caught lying.

Britain's Prince Harry and Meghan the Duchess of Sussex attend the launch of the Nelson Mandela Centenary Exhibition, marking the 100th anniversary of anti-apartheid leader's birth, at the Queen Elizabeth Hall in London, Tuesday, July 17, 2018. (Arthur Edwards/Pool Photo via AP)
Prince Harry and Meghan Markle at the launch of the Nelson Mandela Centenary Exhibition on July 17, 2018 (Picture: AP)
JOHANNESBURG, SOUTH AFRICA - FEBRUARY 1: FILE: After spending 27 years in prison, Nelson Mandela returns home to Soweto and salutes the over 100,000 supporters, jammed into a soccer stadium in the township outside of Johannesburg for a rally in his honor in Johannesburg, South Africa on February 1, 1990. (Photo by Dudley M. Brooks / The Washington Post via Getty Images)
Mandela returns home to Soweto after 27 years in prison on February 1, 1990. (Picture: Getty)
Just two weeks away from being elected President of South Africa, African National Congress(ANC) leader Nelson Mandela visits the rural Xhosa villages where he grew up. The upcoming election will be the first post-apartheid election in the nation's history. (Photo by Andrew Lichtenstein/Corbis via Getty Images)
Just two weeks away from being elected President of South Africa, Nelson Mandela visits the rural Xhosa villages where he grew up (Picture: Getty)
Anti-apartheid leader and African National Congress (ANC) member Nelson Mandela (C, L) and his wife anti-apartheid campaigner Winnie raise fists upon Mandela's release from Victor Verster prison on February 11, 1990 in Paarl. AFP PHOTO ALEXANDER JOE / AFP PHOTO / Alexander JOE (Photo credit should read ALEXANDER JOE/AFP/Getty Images)
Nelson Mandela and his wife anti-apartheid campaigner Winnie raise fists upon Mandela’s release from Victor Verster prison on February 11, 1990 (Picture: AFP/Getty)

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QUNU, SOUTH AFRICA - JULY 17: In this handout provided by the Clinton Foundation, former U.S. President Bill Clinton (R) poses with former South African President Nelson Mandela on the eve of his 94th birthday at his residence July 17, 2012 in Qunu, South Africa. Along with his daughter Chelsea, Clinton met with Mandela for 90 minutes. (Photo by Barbara Kinney/Clinton Foundation via Getty Images)
Former US President Bill Clinton poses with former South African President Nelson Mandela on the eve of his 94th birthday (Picture: Getty)
South African anti-apartheid leader and African National Congress (ANC) member Nelson Mandela (R) shakes hands with British Prime Minister Margaret Thatcher 04 July 1990 on the steps of No 10 Downing Street. Mandela is on a two day visit to the United Kingdom. (FILM) AFP PHOTO/GERRY PENNY (Photo credit should read GERRY PENNY/AFP/Getty Images)
Mandela shakes hands with then British Prime Minister Margaret Thatcher on the steps of No 10 Downing Street on a visit to the UK after his prison release (Picture: AFP/Getty)
JOHANNESBURG - MAY 21: (L) England captain David Beckham awards former South African President Nelson Mandela his own England Jersey on May 21, 2003 at the Nelson Mandela Foundation in Houghton Johannesburg, South Africa. England play Bafana Bafana in friendly match in Durban on Thursday. (Photo by Touchline/Getty Images).
England captain David Beckham awards former South African President Nelson Mandela his own England Jersey in 2003 at the Nelson Mandela Foundation (Picture: Getty)
Former South African President Nelson Mandela tries a Native American pipe given to him as a gift after he spoke at a fundraiser for the Minneapolis Chapter of the National Association for the Advancement of Colored People(NAACP) in Minneapolis, Minnesota, 20 November 2000. . Mandela received the NAACP chapter's Lifetime Achievement Award at the Hilton Hotel in Minneapolis, Minnesota. (FILM) AFP PHOTO Craig Lassig (Photo credit should read CRAIG LASSIG/AFP/Getty Images)
Mandela tries a Native American pipe given to him as a gift after he spoke at a fundraiser for the Minneapolis Chapter of the National Association For The Advancement of Colored People (NAACP) in 2000 (Picture: AFP)
SOUTH AFRICA - JANUARY 01: The South African political leader Nelson MANDELA giving a speech before the African Congress. He was sentenced to a prison life penalty in 1964. Liberated in 1990 Nelson MANDELA shared the Nobel Peace Prize with Frederic DE KLERK in 1993. (Photo by Keystone-France/Gamma-Keystone via Getty Images)
Mandela giving a speech before the African Congress before he was put in prison (Picture: Getty)

Nelson Mandela Inspirational Quotes

‘Real leaders must be ready to sacrifice all for the freedom of their people.’

‘Everyone can rise above their circumstances and achieve success if they are dedicated and passionate about what they do.’

‘Action without vision is only passing time, vision without action is merely day dreaming, but vision with action can change the world.’

‘I am not a saint unless you think of a saint as a sinner who keeps on trying.’

‘Education is the most powerful weapon you can use to change the world.’

‘It always seems impossible until it’s done.’

‘I have cherished the ideal of a democratic and free society in which all persons live together in harmony and with equal opportunities. It is an ideal which I hope to live and achieve. But if needs be, it is an ideal for which I am prepared to die.’

Mandela’s former Robben Island prison-mate Andrew Mlangeni, 93, said their visit shows the monarchy supported the anti-apartheid ‘struggle for freedom’.

‘When I shook their hands I said for the whole month I’m not going to wash my hand – I never thought I would meet royalty,’ Mr Mlangeni said.

It has been a century since South Africa’s first democratically-elected president was born but contemporary South Africa still struggles with race challenges.

A ceremony will take place in Johannesburg to honour his vision for democracy and social inclusion later today.

But outside South Africa, people across the world have taken to Twitter to mark #MandelaDay – an initiative launched by the Nelson Mandela Foundation to celebrate his legacy with a global call to take action against poverty.