Hundreds of Sri Lankan demonstrators have broken through a police barricade at the United Nations offices in Colombo while protesting against the U.N. investigation of alleged war crimes during Sri Lanka's civil war.

The protesters, led by Housing Minister Wimal Weerawanasa, surrounded the U.N. offices Tuesday and blocked the building's entrances and exits.  Police briefly clashed with some of the demonstrators as authorities tried to allow U.N. staff to leave.  U.N. spokesman Farhan Haq voiced "serious concern" about the obstruction of staff which took place despite Sri Lankan government assurances of their safety and security. They also burned an effigy of U.N. Secretary-General Ban Ki-moon.

Last month, the U.N. chief appointed a three-member panel to advise him on whether any human-rights abuses were committed during the final months of the war against Tamil Tiger rebels.

Sri Lankan President Mahinda Rajapaksa has rejected calls for an international probe into possible war crimes, calling it an infringement of the country's sovereignty.  The government has appointed its own commission to examine the conflict and says it will not give visas to members of the U.N. panel.

Analyst Paikiasothy Saravanamuttu, director of the Center for Policy Alternatives in Colombo, says although Tuesday's demonstration was politically-led, there is a measure of public support among those who believe Sri Lanka's human-rights record is being targeted as part of an international agenda.

Sri Lanka's 25-year civil war ended in May 2009 with the defeat of the Tamil Tigers.  The U.N. says 7,000 civilians were killed in the final months of the conflict.