BANGKOK, Thailand (CNN) -- About 20,000 anti-government demonstrators surrounded the Thai government headquarters Thursday evening and set up camp in a bid to oust the prime minister, police said.
Supporters of former PM Thaksin Shinawatra confront riot policemen outside Government House.
The number of protesters outside Government House had grown sharply since the afternoon, said Worapong Tuepreecha, the chief of the Metropolitan Police. He said they set up a stage and tents in front of the building.
The red-shirted protesters are with the United Front for Democracy Against Dictatorship (UDD). They have been protesting since December to oust Prime Minister Abhisit Vejjajiva and his government.
UDD core leader Jatuporn Prompan, also a member of parliament from the Opposition Puea Thai Party, told the Thai News Agency the group's rally would go on for at least a month and that they could even sustain the protest for up to a year if they wish.
The demonstrators gathered Wednesday night at Bangkok's Sanam Luang plaza before marching to Government House on Thursday afternoon, the Thai News Agency reported.
They used a crane to take apart police barriers so they could gain access and surround Government House, the news agency said. They are now blocking several roads and have set up a stage, the news agency said.
Civil servants working at Government House were asked to leave in the early afternoon.
The protesters support former Prime Minister Thaksin Shinawatra, who was replaced by Abhisit in December after some members of the former ruling coalition broke ranks to support him.
Thailand's recent woes date back to a 2006 coup that initially ousted Thaksin. His ruling People Power Party regained office in 2007 elections.
Thaksin's opponents staged more than two months of sit-ins last year, occupying the headquarters of the government and blockading Bangkok's major international airport.
The demonstrations ended December 2 when a court ruled that Thaksin's party was guilty of electoral fraud and threw his brother-in-law out of the prime minister's seat.
Jatuporn said Thaksin might speak to his demonstrating supporters at Government House via video link Thursday evening, the Thai News Agency reported.
-- CNN's Kocha Olarn contributed to this report.