BANGKOK - 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 threat came after a small bomb exploded during the night near the Government House compound, causing no injuries but rattling nerves near the site which thousands of protesters stormed seven days ago.

The activists accuse Samak of acting as a puppet for ousted prime minister Thaksin Shinawatra, who now lives in exile in Britain after the same protest group helped topple his government in 2006.

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.

"We will stop utilities services to government agencies to put pressure on the government to quit and stop damaging our country," said Sawit Kaoewan, secretary of the confederation.

All the union's members have never actually held a strike before, despite threats by their leadership.

One top government official accused the protesters of "guerrilla warfare", urging the workers not to join the protests that have already won support from railway crews who have crippled national train services since Thursday.

"We think that their strategy is guerrilla warfare," the official told reporters, speaking on condition of anonymity.

"The bomb last night is a case in point and we hope that this disruption will not expand," he said.

"We hope that the electricity and water (aren't disrupted). I think all the government team are in negotiation (with unions) to make sure they don't initiate those moves."

The protests had erupted in clashes with riot police on Friday, causing dozens of minor injuries. Activists had also taken their campaign to the provinces, closing key regional airports for two days, including the nation's second-busiest hub on the resort isle of Phuket.

Samak called an emergency session of parliament on Sunday, but the debate failed to produce any plan for ending the protests and the premier angrily rejected calls to dissolve parliament and hold new elections.

"Why are the only solutions house dissolution and resignation? Why can't we choose the third option, which is to show the world that we maintain our democracy," he said.

The leaders of the so-called People's Alliance for Democracy (PAD) have already said that they would not accept a parliamentary solution to the crisis.

In addition to demanding that Samak resign, they want an overhaul of Thailand's system of government, saying only 30 percent of seats in parliament should be elected, with the rest appointed.

PAD gathers most of its support from Bangkok's traditional elite and a portion of the middle class. Its leaders openly disparage the merit of votes cast by the nation's rural poor, who have thrown their support behind Thaksin and now Samak.

Thaksin was toppled by royalist generals in a military coup in 2006, and is now living in exile in Britain to avoid corruption charges at home.

But his allies still fill many top seats in government, and Samak won elections in December by campaigning as Thaksin's proxy.