Protests are being held across Sri Lanka against plans by Western nations to sponsor a UN motion calling for a probe into abuses during the civil war.
Sri Lanka's army defeated separatist Tamil Tiger rebels in May 2009 - both sides have been accused of abuses.
The UN Human Rights Council is meeting to consider a resolution into events during the closing phase of that war.
Sri Lanka's government has rejected calls for an international probe.
It has said it is outraged by support for the UN move in Geneva. Several thousand people including some religious clerics and former military officers marched through Colombo towards the US embassy.
There were also reports that demonstrators in some areas had been coerced to attend.
In the former rebel stronghold of Kilinochchi, in the north, about 500 people gathered and chanted slogans. Although a proportion of these were genuine government supporters, others had been forced to attend by masked motorcycle-riders, one source told the BBC's Charles Haviland in Colombo.
Earlier this month US officials said they would back the UN Human Rights Council resolution, due in March, urging Colombo to investigate war crimes allegations by its own forces.
The government commissioned its own investigation into the war last year and the UN resolution calls on the government to implement its recommendations. The Sri Lankan Lessons Learnt and Reconciliation Commission (LLRC) cleared the military of allegations that it deliberately attacked civilians. It said that there were some violations by troops, but only at an individual level.
But another report commissioned by the UN Secretary General reached a different conclusion, saying that allegations of serious rights violations were "credible" on both sides.
Human rights groups estimate that up to 40,000 civilians were killed in the final months of the war. The government recently released its own estimate, concluding that about 9,000 people perished during that period.