Thousands of protesters have taken to the streets of Bangkok to demand the resignation of Prime Minister Samak Sundaravej and his cabinet.
They occupied the offices of the state broadcaster NBT and surrounded the main government offices.
At one point they forced their way into Mr Samak's offices - prompting him to take shelter in an army base.
The protesters say Mr Samak is merely a proxy for former Prime Minister Thaksin Shinawatra, now in self-imposed exile.
Mr Samak has appeared on television promising decisive action, saying: "We will do everything to bring the situation back to normal."
But he added that it was not yet time to call on the military to restore order.
Anti-government protesters storm the state TV station and surround government offices
'Long holiday for government'
Organisers of the protest - a loose grouping called the People's Alliance for Democracy (PAD) - have been able to galvanise thousands of supporters to their cause.
Tuesday's rally was just one of many street demonstrations they have held so far this year.
"We are now controlling most of the key government offices to prevent them from coming to work," said Sondhi Limthongkul, a leading figure in PAD, according to AP news agency.
"Today, we declare a long, long holiday for the government."
Protesters stormed the offices of the state broadcaster NBT, forcing it off the air. Police arrested several dozen protesters, but then a second wave broke through police lines to reach the station.
Live television also showed about 100 protesters sitting on the lawn of the prime minister's official compound, Reuters news agency said.
This is a mass protest movement with a difference, according to the BBC correspondent in Bangkok, Jonathan Head.
Despite the name, the People's Alliance for Democracy is actually campaigning for an end to democracy, arguing that in Thailand Western-style elections have only led to corrupt and ineffective governments.
Instead, it wants a largely appointed parliament, and a legalised role for the military as a kind of referee in Thai politics, our correspondent adds.
The PAD has already played a central role in Thai politics, beginning three years ago as a movement to bring down Thaksin Shinawatra, then the most powerful elected leader Thailand had ever known.
Its protests set the stage for the coup that ousted him in 2006, and probably helped ensure the legal cases against him went ahead this year, resulting in Mr Thaksin and his wife going back into exile.
Mr Samak is still defiant. His government has a clear majority in parliament, and he insists he retains a democratic mandate from last December's election.
But he seems unable to shake off this determined and apparently well-funded opposition movement, our correspondent adds.
He has also been unable to persuade the security forces to control the protests, raising suspicions that the PAD must have some powerful backers inside the armed forces or among the royalist elite