Thai opposition protesters occupying central Bangkok threatened Tuesday to take the prime minister captive and close down all government offices in an increasingly bold bid to force her from office.

While well known for their blustery rhetoric, the belligerent tone reflects an air of impunity surrounding rally leaders who travel freely around the city despite warrants for their arrest for their role in civil unrest that has left eight dead and hundreds injured.

The protesters, backed by the kingdom's royalist establishment, want Prime Minister Yingluck Shinawatra to resign to make way for an unelected "people's council" that would oversee reforms to curb the political dominance of her billionaire family.

Her supporters say the rallies are a threat to the country's fragile democracy and want the dispute to be settled at the ballot box but the opposition is boycotting a February 2 election.

Demonstrators marched on several key government ministries Tuesday to stop officials from going to work as part of what they are calling a "shutdown" of Bangkok.

Their firebrand leader Suthep Thaugsuban -- a former opposition MP -- vowed from a rally stage in the heart of Bangkok's commercial district to "capture" the premier and her cabinet ministers "one by one" if they do not quit within days.

Suthep himself faces an arrest warrant for insurrection for his role in the seizure of government ministries in November, as well as a murder charge in connection with a military crackdown on opposition protesters that left dozens dead when he was deputy premier in 2010.

But there has been no attempt to detain him and police have been largely invisible during the "shutdown" -- the latest twist of a political crisis that has gripped Thailand since Yingluck's brother Thaksin was ousted in a military coup seven years ago.

The rallies were triggered by a failed amnesty bill that could have allowed Thaksin to return without going to jail for a past corruption conviction.

The billionaire tycoon-turned-politician has strong electoral support in northern Thailand, but he is reviled by many southerners, Bangkok's middle class and members of the royalist establishment.

Democracy or autocracy?

"This is not democracy. It is autocracy... it is a one-man rule," said one of the rally leaders, Satish Sehgal.

"There's massive, rampant corruption in this country. Nepotism. Our objective is to try and get rid of all this," he said after leading several thousand demonstrators to mass outside the customs department.

Demonstrators also temporarily surrounded the ministries of commerce, labour and information and communications technology.

It is a tactic they have deployed several times during the months-long protests, which have so far failed in their goal of forcing Yingluck from office.

A defiant premier urged the opposition -- whose MPs resigned en masse from parliament last month -- to join talks on Wednesday about a possible delay to the election as a way out of the deadlock.

"I am not clinging to office or consolidating my political position," she told reporters. "I am trying to preserve democracy."

Many key junctions remained blocked in the Thai capital with loudspeakers broadcasting bombastic speeches into the city air after protesters launched the shutdown on Monday, causing widespread disruption to Bangkok's central retail and hotel districts.

But the number of demonstrators on the streets appeared to have declined as some returned to work.

The well-organised protest movement has vowed to occupy parts of the city of 12 million people until Yingluck quits, threatening to disrupt the February election which it fears will only return the Shinawatra clan to power.

A hardcore faction of the movement has said it will besiege the stock exchange and even air traffic control if Yingluck does not step down within days, although the mainstream movement has distanced itself from that threat.

The government has not tried to stop the protests, despite warnings that they could take a heavy toll on the economy and local businesses if they drag on.

In a small side street close to the siege of the customs department, some people bemoaned the damage to their livelihoods and expressed support for the embattled government.

"Thaksin helps us. Before he was in government he was already rich," said Supin Nonpayom, a cleaner at a bus terminal, expressing confidence that the Shinawatras are not corrupt.

"They give money to the elderly. They help every group have a better life."