Thai police fired teargas at protesters trying to bring down a caretaker government after a court threw Prime Minister Yingluck Shinawatra out of office and an anti-graft agency indicted her for negligence.
Ms Yingluck's Puea Thai Party still runs the interim government and is hoping to organise a 20 July election that it would probably win.
But the protesters want the government out, the election postponed and reforms to end the influence of Ms Yingluck's brother, former premier Thaksin Shinawatra.
Protest leader Suthep Thaugsuban, speaking to supporters in a city park, urged them to rally outside parliament, the prime minister's offices and five television stations to prevent them being used by the government.
"We will sweep the debris of the Thaksin regime out of the country," said Mr Suthep, a former deputy premier in a government run by the pro-establishment Democrat party.
Mr Thaksin, a former telecommunications tycoon, is vilified by his enemies in the royalist establishment as a corrupt crony capitalist.
But he won the unswerving loyalty of legions of rural and urban poor with populist polices when he was prime minister from 2001 until he was ousted in a 2006 coup.
He lives in exile to avoid a 2008 jail sentence for abuse of power but has been the guiding hand behind his sister's government.
By mid-morning, Mr Suthep had led one group of flag-waving protesters to Government House, the official offices of the prime minister but which have been empty since January. He said protesters would camp outside overnight.
Trouble flared at another protest site when police fired teargas at a crowd of several hundred trying to force their way into a police compound housing a government security group in the north of Bangkok.
The Erawan Medical Centre, which monitors hospitals, said four protesters were taken to hospital after inhaling teargas.
Tens of thousands of the Shinawatras' "red shirt" supporters, angered by Ms Yingluck's ouster, are also on their way to Bangkok for a rally tomorrow.
They are clinging to the hope that the interim government will win the July election and bring the Shinawatras' party back to power.