BANGKOK - A major hospital in the Thai capital evacuated patients and suspended all but emergency surgery Friday after anti-government protesters who occupy a nearby zone stormed in to hunt for security forces they suspected were positioned there.

A group of so-called Red Shirts broke into Chulalongkorn Hospital despite pleas from its director, then withdrew back into their enclave after not finding soldiers or police within the sprawling compound.

The Red Shirts, who began their protests on March 12 in their campaign to force immediate elections, have defied authorities at every turn, entering the Parliament building, laying siege to a telecommunications complex, blocking roads and staging mass motorised rallies in the Thai capital. At least 27 people have died and nearly 1,000 have been injured in outbreaks of street violence.

Security forces have in almost every instance been unable or unwilling to stop the Red Shirt forays, including the incursion into the century-old public hospital, which feared a second break-in Friday.

However, Weng Tojirakarn, a Red Shirt leader and medical doctor, issued a "deep apology" for the raid staged by up to 100 protesters. He called it "inappropriate, too much, and unreasonable".

Authorities sent about 100 police to guard the hospital grounds.

"They can protest all they want but they should not come here, and they should not have prevented us from receiving service," said an angry Purin Supadith, one of many being turned away at the hospital's outpatient department.

In the face of such incidents, Thai pro-establishment activists have demanded military action against the protesters and an end to "anarchy" in the capital.

The re-emergence of the so-called Yellow Shirts - notorious for shutting Bangkok's airports for a week in 2008 - added to the volatility on the streets of Bangkok.

Chamlong Srimuang, a top Yellow Shirt leader, suggested that martial law be imposed, handing over most state functions to the military, and warned that civil war might ensue if the rival "Red Shirt" protesters are not stopped.

The Yellow Shirts represent Thailand's business and bureaucratic elite. Their pervasive influence is deeply resented by the Red Shirts, who largely are drawn from the country's many rural and urban poor.

The unrest is the result of a political standoff over a 2006 military coup that ousted populist Prime Minister Thaksin Shinawatra on corruption allegations. He is a hero to the Red Shirts and is loathed by the Yellow camp. The Red Shirts are demanding current Prime Minister Abhisit Vejjajiva dissolve Parliament, triggering new elections, because they consider his government illegitimate.

The crisis has hurt business in the capital and devastated Thailand's vital tourist industry, which accounts for 6 per cent of the economy.

Parts of Bangkok's commercial heart have become a barricaded Red Shirt protest camp, forcing the closure of some of the city's ritziest malls and hotels. The "occupied zone" flanks Chulalonkgorn Hospital and abuts Silom Road, the capital's "Wall Street" which has become a camp ground for military and police units.

A hospital announcement said patients were being sent to other hospitals or to buildings farther away from the Red Shirts. Almost all outpatient services were being suspended along with surgery, except in emergency cases.

Government forces clashed with Red Shirts on Wednesday as they attempted to hold a rally in a Bangkok suburb. Heavily armed troops fired rifles and threw tear gas at the motorbike-riding protesters and took cover behind terrified commuters' cars. One soldier was killed - apparently by friendly fire - and 18 other people were wounded during the hours-long confrontation.

The crisis has spilled into the diplomatic arena, with Foreign Minister Kasit Piromya censuring some foreign diplomats for meeting last week with Red Shirt leaders.

"We do not want to see that happening again," Kasit told reporters during a visit to Jakarta, Indonesia. Kasit said he had earlier met with Philippine Ambassador Antonio V. Rodriguez, dean of the Bangkok diplomatic corps, to express his concern.

In a note to other diplomats based in Thailand, Rodriguez said Kasit accused some ambassadors of voicing opposition to the constitutional monarchy and criticising the government's handling of the crisis. Kasit was a public supporter of the Yellow Shirt movement before becoming foreign minister.

"These actions have gone beyond the limits of diplomatic practice and were unacceptable to the Thai government," Rodriguez summarized Kasit as saying. "The envoys' opposition to the government and to the monarchy was inappropriate and will not be tolerated."

Thailand's king is nearly universally revered, and laws severely restrict discussion of him.

The United States and European Union both said they have met with opposition figures and called for a peaceful resolution of the crisis.

Though they have been critical of the Red Shirts' tactics, the Yellow Shirts opened the door to mass street protests with months of anti-Thaksin rallies that ended in the coup that ousted him. When pro-Thaksin politicians later came to power, the Yellow Shirts took to the streets again in 2008 -- taking over the prime minister's office for three months and occupying Bangkok's airports for a week. They retreated after Abhisit's arrival in late 2008, but many fear their return could lead to head-on clashes with their rivals.

- AP