Tear gas and water cannon were fired as protesters tried to breach barricades outside Government House.
Activists have threatened to enter key government buildings, including the headquarters of PM Yingluck Shinawatra.
Sunday is the eighth day of protests aimed at unseating Yingluck Shinawatra.
Protest leaders had said it would be the decisive day. They declared it “V-Day” of what they are calling a “people’s coup”.
Two people were killed and dozens more wounded on Saturday as pro- and anti-government groups clashed.
The anti-government Civil Movement for Democracy has announced an all-out assault on the heart of the government, with the aim of replacing it with a “People’s Council”.
It says Yingluck Shinawatra’s administration is controlled by her brother, exiled ex-leader Thaksin Shinawatra.
The Bangkok Post reported that a group of anti-government protesters had taken control of the Thai PBS television station. They told PBS officials to televise speeches made by the protest leaders, the report said.
Police drove back another set of demonstrators at the police headquarters.
Officials denied rumors Yingluck Shinawatra had left the country, but her whereabouts are unknown.
Yingluck Shinawatra had earlier said the government would use minimum force to hold back the protesters.
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