COLOMBO, Sri Lanka (AP) — Hundreds of protesters, led by a government minister, laid siege to the U.N. compound in Colombo on Tuesday, trapping workers inside for hours in an effort to force the world body to cancel its investigation of alleged abuses committed during Sri Lanka's civil war.

The United Nations expressed "serious concern" over the obstruction of its staff, which it said happened despite assurances from the prime minister that the government would ensure their security. More than 125 people were believed trapped in the compound when the protesters first descended on the building in the morning after burning effigies of United Nations Secretary-General Ban Ki-moon.

Police tried to break up the protest in the evening and escorted some of the trapped workers out of the compound, but quickly pulled back after Housing Minister Wimal Weerawansa — who led the protest — ordered them to stop, leaving some U.N. staff trapped inside.

The final staffers — four Westerners — left the building after Foreign Secretary Romesh Jayasinghe, who was apparently negotiating with the protesters, entered and left himself. The demonstration continued outside into the night.

United Nations associate spokesman Farhan Haq confirmed that U.N. staff who were blocked inside the building were allowed to leave their offices by the end of the day. He said that followed U.N. contacts with senior Sri Lanka officials in New York and in Colombo and interventions by the government.

"We take this quite seriously and anything that hinders the movement of staff is a serious concern, and we are trying to make sure that the government will honor its commitments to make sure that our work can be carried out without hindrance," he said at U.N. headquarters in New York.

The Sri Lankan government said in a statement later Tuesday that it allowed the protests because it was obliged to respect people's right to voice their opinion. The U.N. compound was protected and the workers released, it said.

"The government of Sri Lanka expects that the U.N. complex in Colombo would continue to function as normal in the days ahead," the statement said, promising the U.N. staff freedom of movement.

The protesters were demonstrating against a panel formed by Ban last month to examine whether Sri Lankan government forces committed atrocities against minority Tamils when the country's quarter-century civil war drew to a close last year.

The government opposes the panel and has already said it will not issue visas to its members, led by former Indonesian Attorney-General Marzuki Darusman.

Human rights groups have accused the troops and Tamil rebels of deliberately targeting civilians and killing thousands of innocent people in the final months of the war.

The accusations have infuriated top government officials and sparked earlier violent protests outside the Red Cross compound and the British High Commission in Colombo.

In Washington, U.S. State Department spokesman Mark Toner voiced support for the U.N. panel.

The United States supported people's rights to free expression but also "a robust accountability process that will provide a durable foundation for national reconciliation and rule of law in the aftermath of Sri Lanka's decades-long conflict," he told reporters.

As police looked on Tuesday, Weerawansa and a group of ultranationalist Buddhist monks led men waving national flags on a march to the U.N. office. The protesters initially tried to break into the compound, which sits inside a high security zone protected by checkpoints and soldiers, but failed to breach the high walls.

Instead, they held a sit-in, blocking both exists, spray-painting the security camera at the gate — in an apparent bid not to be identified — and preventing employees working inside from leaving.

Between 125 and 200 people were trapped inside the compound, according to a U.N. official inside the building who spoke on condition of anonymity because of the sensitivity of the situation.

Weerawansa demanded the world body disband the three-member investigative team.

"Our armed forces have beaten terrorism in an exemplary manner. We will not allow our soldiers and political leaders to be taken before an international war tribunal," Weerawansa said. "We ask Ban Ki-moon to withdraw this panel if he wants to get the workers and those inside the building out."

Weerawansa said the panel could lead to unfair prosecution of soldiers and political leaders who helped defeat the Tamil rebels after a long and bloody civil war.

Government troops crushed the Tamil Tiger rebels last year, ending their 25-year campaign for an independent state for ethnic minority Tamils. According to the U.N., more than 7,000 civilians were killed in the last five months of the fighting alone.

Sri Lanka has faced growing international criticism of its wartime conduct. Rights groups have said they have photographic and video evidence of abuses and have called for war crimes investigations.

Sri Lanka has denied targeting civilians and has appointed its own reconciliation commission, but that body is not expected to look into the war crimes allegations.

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Associated Press writer Edith Lederer at the United Nations and Foster Klug in Washington contributed to this report.