The protesters were dressed in yellow shirts and headbands and carried national flags and portraits of the revered King Bhumibol Adulyadej.
This is the final showdown
Protest leader Somsak Kosaisuk
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.
House speaker Chai Chidchob said the session was cancelled because most MPs had been unable to get into the building - and appealed for calm.
"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."
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.
Thousands of police were out on the streets on Monday, but police chiefs said they were better prepared this time.
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.
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.
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."
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 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.
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.
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