Cannot play media.You do not have the correct version of the flash player. Download the correct version
Tens of thousands of Thai opposition supporters have rallied in Bangkok to press the government to step down.Protest leaders have given the government until Monday to call fresh elections.
Prime Minister Abhisit Vejjajiva say he will not resign. About 40,000 troops and police have been deployed.
The "red shirt" demonstrators are mainly supporters of former Prime Minister Thaksin Shinawatra, who was ousted in a military coup in 2006.
Many of the protesters have come to the capital from Mr Thaksin's power base in the rural north of Thailand.
The BBC's Rachel Harvey, at the demonstration, says there are about 100,000 protesters. Red shirt organisers said hundreds of thousands would come.
Many were gathered in front of a stage in central Bangkok on Sunday to hear their leaders make the demands.
One of the opposition leaders, Veera Musikapong, told cheering crowds: "Reds over the land call on the government to return power to the people and to dissolve the house immediately. We will hold out here and wait for an answer within 24 hours."
The government of Thailand says it has no intention of standing down.
Organisers have promised a peaceful demonstration, but said that if the government refused to quit they would step up their campaign.
The protesters, led by the United Front for Democracy against Dictatorship (UDD), say Prime Minister Abhisit had come to power illegitimately with the backing of the military and the Bangkok-based elites.
The military has been given extra powers to impose curfews and restrict numbers at gatherings if necessary.
The last major protests, in April last year, turned violent, with two deaths and dozens of people injured.
But this may be the red shirts' last chance to reverse Thailand's political direction, says our correspondent, with the movement tiring and probably running low on funds.
Thailand has been in political turmoil since 2006 when yellow-shirted anti-Thaksin protesters began demanding the then prime minister step down over corruption accusations.
He was later forced out by a coup, but when his allies came back to power in 2008, his opponents occupied the prime minister's office for three months and seized Bangkok's two main airports for a week.
Mr Thaksin is now living in self-imposed exile in Dubai after receiving a two-year sentence in his absence for abuse of power; his supporters says that case was politically motivated.