Thousands of protesters took to the streets of Thailand's capital, Bangkok, to demand the resignation of the government.
The demonstrators say that Prime Minister Samak Sundaravej's government is a proxy for Thaksin Shinawatra, who the military overthrew in 2006.
There were some scuffles with police as the mass of protesters came up against barricades in central Bangkok.
A column of protesters appeared to take the security cordon around Government House by surprise as they came up behind them.
The demonstrators were led by the People's Alliance for Democracy (PAD) - mostly made up of middle-class Bangkok residents who have been protesting against the government since May.
Government moves to amend the military-backed constitution have angered PAD supporters who say the ruling People Power Party will benefit.