BANGKOK (AFP) -- Embattled Thai Premier Thaksin Shinawatra vowed to stand firm against mounting pressure for his resignation as protesters on Monday called for civil disobedience in a bid to force him out of office.

"I am not going to be the prime minister who resigns after a small bit of pressure against me," Thaksin told an audience of thousands of supporters in the northeastern city of Khon Kaen late Sunday.

"Other prime ministers resigned in the past when conflicts deepened, but I won't. I don't want to set a bad precedent," he said.

Thaksin, battling allegations of corruption and abuse of power, has been the target of hordes of protesters who demanded his resignation at four mass rallies in as many weeks. But he is still very popular among rural voters.

Over 50,000 protesters gathered Sunday at Bangkok's Sanam Luang field near the royal palace, the same venue where Thaksin drew some 150,000 supporters on Friday as he kicked off his campaign for April 2 snap elections.

Like previous demonstrations, Sunday's rally was noisy but peaceful. Yet protesters marched to Government House, the seat of Thailand's government, in a show of force to pile the pressure on Thaksin.

Parnthep Pourpongpan, a spokesman for Sondhi Limthongkul, a key member of the anti-Thaksin People's Alliance for Democracy, said Monday the group would hold daily protests until the premier quits, calling for civil disobedience.

"We will meet every day at 4:00 P.M. (at Sanam Luang) until Thaksin resigns. We will continue our efforts peacefully and never give up until he quits, which is our main objective," he said.

Parnthep said Chamlong Srimuang, an anti-Thaksin leader who heads the banned Buddhist sect Santi Asoke, could camp at Sanam Luang for one year in a campaign to oust Thaksin.

Chamlong, Thaksin's one-time mentor, led bloody demonstrations against the military government in 1992. The uprising killed at least 52 people and saw the disappearance of 175 more, according to rights groups.

Panitan Wattanayagorn, a visiting professor of international relations at Johns Hopkins University, said the anti-Thaksin groups hoped civil disobedience would broaden public support and increase pressure against the premier.

"There is a deadlock between the prime minister's supporters and anti-Thaksin groups," Panitan said.

"Protesters are adopting civil disobedience as a new measure to broaden public support and add more pressure against Thaksin. If their tactics are seen as confrontational or violent, it will alienate the public," he said.

Meanwhile, police said they were beefing up security at Government House and other public buildings amid the prolonged political turmoil.

"We have maintained at least 400 police officers at Government House as we know that anti-government rallies will continue," said national police spokesman Ajirawid Subarnbhesaj.

"As protests continue, both demonstrators and police officers will be stressed out and we just don't know what will happen next," the police spokesman said.

Faced with the weekly street protests, Thaksin dissolved Parliament on February 24 and called snap elections three years early, but leading opposition parties have decided to boycott the April polls.

Analysts say the opposition boycott threatens the legitimacy of the polls and could throw the kingdom deeper into political crisis.

Thaksin won his second term in office last year with an absolute majority in Parliament, an historic victory that made him appear invincible.

But simmering discontent among Bangkok's middle class bubbled over in January when his family sold their shares in Shin Corp, the telecoms giant Thaksin founded before entering politics, for 1.9 billion dollars, tax-free.