BANGKOK -- Thailand's embattled prime minister declared a state of emergency in the capital Tuesday after thousands of his opponents and supporters clashed in the worst street violence here in more than a decade.

One person was killed and dozens were injured, some of them from gunshot wounds, as a week of mass protests calling for the resignation of Prime Minister Samak Sundaravej spilled over into bloodshed overnight.

Samak had previously said he would not use force to push out the thousands of protesters who have occupied the main government complex since last week, but after the violence said it was now time for them to go.

"They must be moved from the Government House," Samak told a nationally televised news conference. "I had no other choice but to declare a state of emergency in Bangkok in order to solve the problem for once and for all."

But a leader of the People's Alliance for Democracy (PAD), a well-organised movement that has been pushing for the premier to stand down after just seven months in office, urged his supporters to stay put.

Thailand's biggest union Monday called for a strike to support anti-government protesters who are squatting in the main government complex to demand the resignation of Premier Samak Sundaravej.

The 200,000-strong State Enterprises Workers' Relations Confederation called for a strike Wednesday to add pressure on Samak, saying they would cut power and water supplies to government agencies. PAD supporters stormed Samak's Government House complex one week ago, and thousands are still squatting on the grounds.