Mr Obama is the 44th US president
Listen Listen to the inauguration special (180 mins)
Listen Listen to the full speech (18 mins 31 secs)
Barack Obama has been sworn in as the 44th president of the United States.
BBC World Service brought live coverage of the historic event from the US capital, with a special three-hour programme presented by James Coomarasamy and Ros Atkins in Washington, together with online updates by Ben Sutherland.
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1900GMT: That's it from our special coverage of the inauguration. You can follow continuing coverage on BBC News.com, but from us at the World Service, that's all - see you in four - or maybe eight - years' time.
1844GMT: It turns out John Roberts, the chief justice, was possibly the one who stumbled over his words at the moment of inauguration - and that actually the reason Mr Obama's repetition of the oath of office was different was because Mr Obama was using the correct words.
1839GMT: In Caracas, Will Grant gets a view from Luis and Paul, modern languages students in Venezuela. "It's clear his priorities are at home in the US. I don't think it's very special that he has become president," Paul says. Luis, however, is hopeful Obama will open up relations with Venezuela. "Surely there will be a calmer relationship between the two countries," he says. "I think that Obama will really bring change to the relationship with the world."
1833GMT: The new president is now attending his inaugural lunch.
1828GMT: A thought-provoking text arrives. "Had Barack Obama been in Kenya rather than America, his chances of getting the top job would have been pretty low - which makes me feel bad for Kenya," it says.
1824GMT:Lou, an army veteran from New Jersey, tells Ros Atkins he hopes the world is not expecting too much from Obama's America. "We will fulfill the role that we had prior to eight years ago," he suggests. "Everybody has to step up around the world; help us and we can help each other."
1820GMT:Jamie Coomarsamy on Capitol Hill considers Obama's foreign policy - and says it has not been particularly forthcoming. It is not thought Hillary Clinton will be confirmed as Secretary of State today, as had been previously speculated.
1813GMT: The crowd is thinning out near the national mall, Ros Atkins reports. It has also started snowing - although not as heavily as it famously did in 1909, when William Taft was forced to take the oath in a blizzard.
1800GMT: Here are the final words of the speech in full: "Let it be said by our children's children that when we were tested we refused to let this journey end, that we did not turn back nor did we falter; and with eyes fixed on the horizon and God's grace upon us, we carried forth that great gift of freedom and delivered it safely to future generations. Thank you, God bless you and God bless the United States of America."
1753GMT: From the mall, Ros Atkins has met four African-American women. All agree that Aretha Franklin stole the show with her performance of the national anthem - while wearing a quite extraordinarily large bow.
1748GMT: Diplomatic correspondent Jonathan Marcus gives his first reaction to the speech. He emphasises the references to "smart power" - how he suspects the new administration will harnessing diplomatic and political as well as military force, together with force of example and the attraction of America's society and values.
1727GMT: The new President finishes his speech with the words "Thank you, god bless you, and god bless the United States of America." He receives thunderous applause.
He said that this moment "will define a generation." The speech touched on a wide range of subjects. He said "that we are in the midst of crisis is now well understood" - a crisis that includes being at war "against a far-reaching network of violence and hatred", a "badly weakened" economy, and health care that is "too costly."
Addressing the economy specifically, he said that "this crisis has reminded us that without a watchful eye, the market will spin out of control."
He called for "a new era of responsibilty at home and abroad." Touching on foreign policy, he said that "America is a friend of each nation, of ecery man, woman and child who seeks peace and dignity, and we are ready to lead once more."
1707GMT: Barack Obama begins his speech - "one of the most anticipated inauguration speeches in history," our correspondent Ros Atkins says.
1705GMT: Everyone stands for the Chief Justice of the United States, John G Roberts Jnr. He asks the Senator if he is prepared to take the oath. "I am," he responds. He stumbles a little, but agrees to "preserve, protect and defend the constitution of the United States." Barack Obama is now the President of the United States.
1700GMT: A number of America's top classical musicians begin playing a composition called Air And Simple Gifts, written for the occasion by John Williams - best known for his work on films such as Star Wars and Indiana Jones.
1658GMT: Vice-President elect Joe Biden is sworn is by Justice John Paul Stevens - and now formally loses the "elect."
1655GMT:Aretha Franklin begins the national anthem; the sun shines behind the American flags. It is a very evocative image.
1653GMT: Pastor Rick Warren begins his invocation. "To you lord, we now commit our new President, his wife Michelle, and his daughters," he says.
1648GMT: Senator Dianne Feinstein begins the ceremony, describing the day as "another line in the solid stone of history."
1643GMT: And here's the man himself. The band strikes up and Barack Obama - introduced as "Barack H Obama" - walks down the steps. The ceremony is about to begin.
1638GMT:Joe Biden, the new vice-president, heads down the steps and there is a huge cheer. Very shortly afterwards, TV pictures show Barack Obama entering the building.
1636GMT: The Bush White House team - not just the 43rd president, but also his cabinet and Dick Cheney, in a wheelchair after he hurt his back packing items as he prepared to move out of the White House - steps into the Capitol arena. There is a smattering of light applause.
1631GMT: And hot on the heels of Michelle is - in the last half hour of his presidency - George W Bush.
1630GMT: And just a few minutes later, Michelle Obama arrives. There has been some speculation about what she would wear - so just so you know, it's a gold-embossed dress with matching jacket.
1624GMT: The latest arrivals are Mr Obama's daughters Malia and Sasha.
1619GMT: It's the turn of Jimmy Carter - Democratic president from 1976 to 1980 - to enter the specially-erected arena outside the Capitol.
1617GMT: Here come the Clintons - Bill, who was in office from 1992 to 2000, and his wife Hillary - who of course ran against Mr Obama for the Democratic nomination.
1615GMT: And dressed in matching purple outfits are George HW Bush and his wife Barbara.
1608GMT:Al Gore - who came so close to the presidency in 2000, before losing out to George W Bush in Florida - has arrived.
1600GMT: Our three hours of special programming begins, live from Washington.
Listen Click here to listen live
1548GMT: News services AFP and Reuters report that a threat has been made on the inauguration. Get the latest on this from the BBC News website.
1525GMT: With the official ceremony set to begin shortly, James Coomarasamy on the steps of the Capitol building has the details of the timetable for the next two hours. The US Marine Band will begin, before the living former presidents, including George HW Bush and Bill Clinton, arrive. There will then be an address from pastor Rick Warren - possibly the most controversial part of the occasion, as he is a social conservative with outspoken views on, for example, gay marriage. The singer Aretha Franklin will then perform - before finally, just before midday, Chief Justice John Roberts will swear Mr Obama in as the 44th president.
1510GMT: The demands on the new president are wide and varied. If you missed them, you can listen again to a collection of open letters to Mr Obama from five writers and poets, expressing their hopes and concerns:
Listen to our open letters to the new president
1505GMT:The BBC's Andy Gallagher in Washington describes the scene as he looks out at "what must be the single most populated point on the planet", across "a sea of people waving flags and scarves." Security involves not just one, but several rings of steel around the city, with no-one allowed in with umbrellas or bags. Nevertheless - and despite the cold - the atmosphere is one of "an enormous party," he adds.
1455GMT:A little bit of history from the World Service archives on Newshour. Here's black civil rights leader Martin Luther King - the man in whose name Barack Obama has urged the nation to unite - speaking to the BBC's Bob McKenzie back in 1964:
Listen Listen to Martin Luther King (1 min 5 secs)
Also, you can listen again to our interview with Reverand King's sister, Dr Christine King Farris, on what Mr Obama's inauguration means to her:
Listen
Listen to Dr Christine King Farris (3 mins 11 secs)
1420GMT:An opinion from China now - the country many believe will be the next superpower to challenge America's hegmony - and likely as not, to do so during Obama's term in office. A student in Beijing tells the BBC: "I'm so surprised that a black person could win the US presidental election. I always think that racial discrimination in the US is deep-rooted. However, Obama's winning the election has changed my perception. On America's policy towards China, I hope that Obama should handle trade dispute with care - as China should not be blamed entirely for US's trade deficit."
1410GMT:Ros Atkins is out on the mall, gathering opinions. The word on everyone's lips seems to be "unity." Meanwhile, we are told that Mr Obama will speak for 17 minutes when he makes his address.
Listen
Listen to Ros Atkins (3 mins 20 secs)
1400GMT:Some more emails from our language services. "The world is fascinated by the fact Obama is an African descendent," says Malbert Brinner, from Brasilia via the BBC's Brazilian website. "US$160 million will be spent in a Hollywood-style inauguration show of a mysterious president, of whom we do not know the mentors and sponsors. In fact we only know his vain promises of change made during the election campaign. Let's wait and see his actual politics."
Fernanda Serpa in Sao Paulo is more optimistic, however: "the big and main hope of the American people is about the end of the wars, and the Barack Obama has a different approach to what president Bush has called terror," he says.
1327GMT: The BBC's Robin Lustig has been gauging the mood in Alabama - the state whose governor once said "segregation now, segregation tomorrow, segregation forever" - ahead of Barack Obama's inauguration in Washington as the first African-American president:
Listen Listen to Robin Lustig (9 mins 27 secs)
1300GMT: Some thoughts brought to you via the BBC's Hindi service. "Barack Obama's inauguration is going to realise Martin Luther King's decades old dream," says Ravi Shankar Tiwari, from Bihar in India - who also argues that the "world's biggest challenge is at the moment is terrorism; I hope Obama will deal with the issue very effectively."
But Peter Parkash, living in Hornsby, Australia, warns that "India needs to watchful as Obama's Kashmir policy is anti-India. His policy can become an obstacle in achieving cordial Indo-Us relations. Obama's policy on Pakistan is also soft which not a great sign for India."
1245GMT: Emails have been pouring into our language services from all around the world. Hadi in Tehran, for example, tells BBCPersian.com that "The best thing Obama can do is to stop the harsh language in America against Iran and Iranians, facilitate visa for Iranian nationals and treat them with respect. The hardliners in Iran (especially the Ahmadinejad administration) use the hostile tone by Bush and his government to their own benefit."
However,Hussain in the city of Qom is more sceptical regarding any ability of Mr Obama to change US-Iranian relations. "There are third parties and some other factors involved here," he says. "Imagine the day that (Iranian officials) tell the people they have become friends with the "Great Satan", then whom would they blame for all their problems?"
What do you think? Have your say on BBC World Service's World Have Your Say website.
1230GMT: Foreign leaders have been giving their thoughts ahead of Mr Obama's historic address. Liberia's president Ellen Johnson Sirleaf tells us she is as "excited as people the world over":
Listen Listen to Ellen Johnson Sirleaf (2 mins 10 secs)
1215GMT: Our correspondents have been gathering the views of some of the visitors from around the world as they descend on Washington ahead of the inauguration - including a British couple:
Listen Listen to Brits in Washington (36 secs)
Meanwhile we have also been measuring the global mood by talking to expat Americans living in London, Delhi and Johannesburg:
Listen Listen to expat expectations (12 mins 48 secs)
1200GMT: There are four hours to go until Barack Obama begins his address. The crowds are already beginning to gather in anticipation - but what exactly is he likely to say? We asked famed Washington author and speechwriter David Frum:
