Thousands of demonstrators have surrounded Thailand's parliament building, prompting the day's parliamentary session to be cancelled.
Tens of thousands of demonstrators have surrounded Thailand's parliament building in what they say is a "final battle" to topple the government.
Protesters have been occupying the government compound in the capital, Bangkok, for months.
The protesters have been occupying the government compound in the capital, Bangkok, for months.
They say their mass protest is a "final battle" to topple the government which they say is a proxy for former, exiled Prime Minister Thaksin Shinawatra.
The protests are led by the royalist People's Alliance for Democracy (PAD), which claims the government is corrupt and hostile to the monarchy.
The protesters belong to the opposition People's Alliance for Democracy (PAD).
The rallies forced the day's parliamentary session to be cancelled.
The current Prime Minister, Somchai Wongsawat, who has been attending the Asia Pacific Ecocomic Co-operation (Apec) summit in Peru has said he has no intention of resigning.
The current Prime Minister, Somchai Wongsawat, who has been attending the Asia Pacific Ecocomic Co-operation (Apec) summit in Peru, has said he has no intention of resigning.
Stalemate
Violence fears
Three months after it stormed the prime minister's offices and set up a fortified camp there, the People's Alliance for Democracy still has not achieved its objective, the BBC's Jonathan Head reports.
Police said around 18,000 demonstrators had taken to the streets, blocking roads leading to parliament. Some groups had marched on the police headquarters and the finance ministry.
The PAD has helped unseat one prime minister, but the government, led by allies of Mr Thaksin, remains in office - although largely paralysed by the protests.
The protesters were dressed in yellow shirts and headbands and carried national flags and portraits of the revered King Bhumibol Adulyadej.
Now the PAD says it hopes to bring enough demonstrators to key locations in the capital to force the government from power, or at least provoke a military takeover, our correspondent says.
This is the final showdown Protest leader Somsak Kosaisuk
Many thousands are already surrounding the parliament, where a number of important regional agreements were due to be discussed.
MPs were forced to cancel a parliamentary session, in which they would have discussed a number of important regional agreements. Their original plans to debate proposed changes to the constitution had prompted the PAD to call Monday's mass protest.
When the PAD tried this last month, they sparked off the most violent clashes in Bangkok seen for 16 years.
House speaker Chai Chidchob said the session was cancelled because most MPs had been unable to get into the building - and appealed for calm.
Once again thousands of police are confronting them - but this time the police say they are better prepared.
"I promise that there will be no violence today, not a single drop of blood will be seen," he told parliament radio. "I ask for all sides to stop the movement now. If you love the king, please return home."
The police have been offered 15 fire engines to use for crowd control, instead of the explosive tear gas grenades which caused such severe injury during the last demonstration.
There are fears that the protests will see a repeat of the clashes in October, which left two people dead and some 500 injured. It was the worst violence in Bangkok for 16 years.
PAD protesters, who police numbered at around 18,000, were also surrounded by their own armed security guards.
Thousands of police were out on the streets on Monday, but police chiefs said they were better prepared this time.
Final push?
They have been offered 15 fire engines to use for crowd control, instead of the explosive tear gas grenades which caused such severe injury during the last demonstration.
Protest leaders started marshalling crowds on Sunday.
The BBC's Jonathan Head in Bangkok said the protests have been peaceful so far, but added that there are an awful lot of people out on the streets and the PAD leaders are using fiery rhetoric in their speeches.
The key protest leader Chamlong Srimuang was reported as saying over the weekend that if the latest push fails to oust the government, the alliance would stop its protests.
One, Somsak Kosaisuk, told protesters: "This is the final showdown. We have been here a long time. We have been patient. But they have robbed the country to the point where it can't take it anymore. I promise you that will soon end."
Police said about 10,000 pro-Thaksin supporters had descended on a Buddhist temple just outside Bangkok to support the government.
Long-running grievances
The PAD is aiming for a repeat of 2006, when its protests led to a military coup that unseated former Prime Minister Thaksin Shinawatra.
They accused the telecommunications billionaire of being corrupt and abusing his power. Prime Minister Somchai Wongsawat is related to Mr Thaksin
Mr Thaksin's allies won the first post-coup elections in late 2007. The PAD accuse the government of being a proxy for the former prime minister.
Police said about 10,000 pro-Thaksin supporters had descended on a Buddhist temple just outside Bangkok on Sunday to support the government.
The PAD wants to replace Thailand's one-man, one-vote system with one in which some representatives are chosen by professions and social groups rather than the general electorate.
The PAD wants to replace Thailand's one-man, one-vote system with one in which some representatives are chosen by professions and social groups rather than the general electorate.
The new government says it wants to start negotiations with the PAD.
The new government says it wants to start negotiations with the PAD.
But it is also pushing ahead with controversial plans to amend the constitution - a key grievance of the protesters who see it as part of a plan to rehabilitate former PM Thaksin.
But it is also pushing ahead with controversial plans to amend the constitution - a key grievance of the protesters who see it as part of a plan to rehabilitate former PM Thaksin.
Mr Thaksin, Mr Somchai's brother-in-law, was forced from office in a military coup in 2006 and remains in exile overseas.
Mr Thaksin, Prime Minister Somchai's brother-in-law, remains in exile overseas. He was sentenced in absentia by the Supreme Court in Thailand last month to two years imprisonment on a corruption charge. In July, his wife received a three year jail sentence for tax fraud.
Are you in Bangkok? Have you been affected by the protests around the parliament? Send us your experiences.
