The protesters have vowed to keep rallying until the government steps down
Tens of thousands of Thai opposition supporters have rallied in Bangkok to press the government to step down.
They gave Prime Minister Abhisit Vejjajiva until Monday to call fresh polls and vowed to demonstrate across the capital if he refuses to do so.
The government insists it will not stand down and has tightened security.
The "red shirt" demonstrators are mainly supporters of former Prime Minister Thaksin Shinawatra, who was ousted in a military coup in 2006.
AT THE SCENE
Rachel Harvey, BBC News, Bangkok
The atmosphere here in the centre of the rally is very noisy, very boisterous, but it is jovial.
One of the challenges for the red shirt leadership is going to be to try to maintain that sense of order.
There are thousands of police on duty in full riot gear but they are keeping a low profile.
The other key test for the red shirt leadership - at the same time as maintaining order - is: Can they put enough pressure on the government to force it to give in to their demands, step down and call fresh elections?
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."
Another protest leader, Nattawut Saikua, said people would step up the campaign and march throughout Bangkok if the government does not comply.
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.
Tens of thousands of riot police and soldiers have been deployed outside Government House and other strategic points.
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.
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