Protest leader Suthep Thaugsuban had earlier refused a government offer to delay the election if protesters and opposition supporters agreed not to boycott another one that could be held within months.

“This isn't about compromise,” said Mr Suthep, a former deputy prime minister in a military-backed government.

“The people (protesting) will never go home because what the people want is political and national reform,” he said.

The election called for February 2 remains in doubt after the Election Commission, the agency that organises elections, called for it to be postponed following months of sometimes violent unrest, including the shutdown of parts of Bangkok and the occupying of key ministries.

In Thailand advance voting opens a week earlier that the main polling day.

“As for the February 2 election, I don't believe it can be held,” Theerarojwit Theerarowit, an election commissioner told reporters.

“It won't be able to be held because if there are elections on that day people could get hurt and (the Election Commission) doesn't want to see people get hurt,” he said.

Prime minister Yingluck Shinawatra is due to meet with the commission on Tuesday to discuss whether the election can go ahead on February 2.

Several of her ministers have said delaying the poll would be futile if another called within months was also boycotted or blocked by protesters.

The main opposition Democrat party is refusing to participate in the election and has not registered candidates.

Protesters moved before dawn to blockade dozens of 50 polling sites across Bangkok despite a warning their actions were illegal.

Election officials arriving to prepare for voting at 6am at Bang Kapi, a Bangkok district, were blocked by protesters already there.

Many polling stations in southern provinces that are strongholds of the Democrat party were also blockaded.

Protesters set-up a rally stage outside a police training school in southern Surat Thani where voting was scheduled to be held.

Polling officials were instructed to close voting at the first sign of any trouble and take away ballot papers.

“Any move seen as obstructing advance voting on Sunday or February 2 is illegal, subject to jail sentence or a fine or both,” said foreign minister Surapong Tovichakchaikul in a televised address.

But police have orders only to intervene to stop violence and have not moved to arrest Mr Suthep and other protest leaders who have been charged with treason.

The government fears the country's powerful military will take-over if violence escalates.

About 49 million of Thailand's 64 million people are eligible to vote and 2.16 million of them have applied to vote early.

Australia's Department of Foreign Affairs advised Australians to exercise a high degree of caution because protests at polling stations were likely to result in violence in some locations.

The government on Wednesday imposed a state of emergency giving security forces wide powers, including banning gatherings of more than five people.

But the government said it would not use the powers to crackdown on protesters occupying key intersections in Bangkok who have been marching to government offices each day in an attempt to paralyse the government.

The unrest is the latest episode in an eight-year conflict that broadly pits opposing elites against each other, one supported by Bangkok's middle class and southern Democrat supporters and the other backed by mostly poor rural people in north and north-eastern provinces.

Protesters are demanding that Ms Yingluck and her elder brother Thaksin Shinawatra quit politics and allow the setting-up of a “people's council” to run the country for up to two years.

They accuse Mr Thaksin, a billionaire who lives in Dubai to avoid a jail sentence for corruption, of running the country with Ms Yingluck as his puppet.

Since the protests began in November nine people have been killed, almost 600 injured, the country's currency, stock market and investment have fallen sharply and tourist arrivals have collapsed at the peak of the tourist season.