About 100 Sri Lankan demonstrators led by Buddhist monks have protested outside the UN office in Colombo against the visit of Navi Pillay, the UN High Commissioner for Human Rights.
The pro-government activists accused the international community of drumming up false allegations of war crimes in 2009 during the final months of a separatist conflict.
They called on Pillay to leave the country and stop criticising Colombo's human rights record.
"She should visit countries like Iraq and Afghanistan which have become slaughter-houses," said Ittekande Saddatissa, a Buddhist monk.
"We will not hesitate to muster the protest of the entire citizenry against the UN if attempts are made to fabricate diabolical lies against Sri Lanka," the activists said in a petition. The activists handed the petition to a senior UN official before dispersing.
Pillay, a South African of Indian Tamil origin, is in Colombo for seven days. She will meet Sri Lanka's president, Mahinda Rajapaksa, and human rights activists.
Pillay will also travel to the country's former war zone where Tamil civilians were allegedly killed as the quarter-century war ended in 2009.
She will report her findings to the UN Human Rights Council next month. Pillay, who has previously been accused by Colombo of overstepping her mandate, said on Sunday that she was holding Colombo to human rights standards agreed by all nations.
"I have not come to criticise. I have come to raise human rights concerns," she said.
The Sri Lankan government battled separatist Tamil Tiger fighters from 1983, finally defeating them in 2009.
In March, the UN Human Rights Council passed a resolution calling on the Sri Lankan authorities to thoroughly investigate alleged war crimes committed by government troops and Tamil fighters.
The UN says it has "credible allegations" that Sri Lankan troops and rebels both carried out atrocities and war crimes and up to 40,000 civilians were killed in the final months of the war.