Samak Sundaravej was appointed Thailand's prime minister in Januar

Mr Samak earlier held an unexpected meeting with King Bhumibol Adulyadej, who normally stays above politics.

Thousands of protesters continue to occupy government offices in Bangkok, three regional airports remain closed, and rail travel has been disrupted.

The protesters accuse the government of being a front for the exiled former Prime Minister, Thaksin Shinawatra.

Mr Thaksin was ousted by a military coup in 2006 and has fled the country to avoid facing trial over corruption charges.

'Threats'

As the political unrest continued across Thailand on Friday, Mr Samak flew to the coastal town of Hua Hin to meet King Bhumibol at his palace there, officials said.

The BBC's Jonathan Head in Bangkok says the king normally keeps out of political squabbles, but what started as a last-ditch attempt to force out the prime minister by a die-hard group of protestors, has in the space of 24 hours turned into a national crisis.

Mr Samak was initially praised for his restraint in dealing with the protests

Earlier, military and government sources told the BBC that the army commander, Gen Anupong Paochinda, who until this week had stood by the prime minister, had told him he had no choice but to resign.

But in a televised ceremony honouring the royal family in the capital on Saturday, Mr Samak again reiterated his refusal to step down, saying he had been elected to office.

"I will never resign in response to these threats," he said, to cheers from the crowd.

The protests against Mr Samak's government are being led by a conservative group called the People's Alliance for Democracy (PAD).

On Tuesday, thousands of PAD supporters took to the streets of Bangkok and forced their way into government buildings.

Mr Samak was initially praised for his restraint in dealing with the protests, but he suddenly found his position weakened on Friday when the police pulled back and the triumphant protest leaders were left in control of the government complex.

The PAD was originally formed in the months before the 2006 coup, to demand Mr Thaksin's resignation. It has re-emerged now that the country is being led by his former ally Mr Samak.

It has a passionate following in various parts of the country, especially Bangkok, and some powerful backers among the elite, our correspondent says.

But it has little support in most of rural Thailand, which voted strongly for Prime Minister Samak, and Mr Thaksin before him.