Opponents of Thailand's new government have rallied in central Bangkok and called for fresh elections,.
The red-clad protestors massed a day before Mr Abhisit was due to give his maiden policy speech to parliament, saying they would not give up until the government, that came to power two weeks ago, holds fresh elections.
Most of the protestors gathered at a central Bangkok parade ground and organisers said they would move to parliament overnight.
An advance guard of several hundred had already blocked a key road outside the legislature.
Police said more than 20,000 protestors had gathered while organisers said the figure was 50,000. More than 3,000 unarmed riot police were on duty, handing out leaflets urging peaceful protests.
A huge stage at the parade ground near the royal palace was backed with a red banner saying ‘No confidence in Abhisit Vejjajiva’, while protestors waved signs saying ‘We Love Thaksin’.
Mr Thaksin was ousted in a military coup in 2006 and remains in exile to avoid a jail sentence for corruption.
Organisers said he might make a telephone address to the rally.
The demonstrations bring Thai politics full circle after a year of turmoil, with Thaksin loyalists now using the same tactics that helped rival protestors to bring down a government led by the tycoon's allies.
Oxford-educated Mr Abhisit, the head of the Democrat Party, won a parliamentary vote to become prime minister on 15 December, less than two weeks after a court dissolved the former ruling People Power Party that was loyal to Mr Thaksin.
