The centre of Bangkok has been flooded with more than 100,000 protestors, who have threatened to sit there for a week unless there is a new election in Thailand.
The red-shirted supporters of former prime minister Thaksin Shinawatra issued an ultimatum to the government, to dissolve parliament or face chaos in the capital.
The turnout showed just how much influence Shinawatra commands, despite being ousted from power in 2006, convicted of corruption and going into exile.
On a video link, he urged the crowds to press ahead in their attempt to change Thai politics from an elite era to a democracy.
The red-shirted protestors claim the current premier, Abhisit Vejjajiva, was installed with the support of the military after the twice-elected Shinawatra had been toppled.
They see his rule as part of an unelected elite in Thailand.
Last April, Thaksin supporters sparked the worst street violence in Thailand for 17 years. This time, organisers have stressed non-violent protests.