The protesters blocked the entrance to Samak's office, occupied the government-owned NBT television building and raided several ministries and the capital police headquarters, reported Bernama, the Malaysian news agency.
NBT remained on the air, transmitting from mobile units around the capital and police didn't intervene to quash the protest.
Samak issued a statement saying he remained in control at Supreme Command Headquarters in Bangkok and army Chief Gen. Anupong Paojinda said the military wouldn't participate in a coup, Bernama reported.
The same group of protesters, the People's Alliance for Democracy in 2006 organized rallies that led to the ouster of former Prime Minister Thaksin Shinawatra in a bloodless coup.
This wave of PAD rallies began May 25 with hundreds of protesters disrupting business and government in Bangkok. An estimated 30,000 protesters participated in Tuesday's rally, The New York Times reported.