Tens of thousands of anti-government protesters have gathered in Thailand's capital Bangkok to press the government to step down and hold new elections.
About 40,000 soldiers and police are on the streets in case of trouble.
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.
They have been making their way to Bangkok since Friday.
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 to hear their leaders make the demands signalling the official beginning of the protest, says our correspondent.
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 Vejjajiva had come to power illegitimately with the backing of the military and the Bangkok-based elites.
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.
