At least five cabinet ministers led crowds in Colombo where they chanted slogans against Washington`s plan to move a resolution against Sri Lanka at the UN Human Rights Council which opened in Geneva Monday.
"We are here to protest the action the US is trying to take against us," Energy Minister Champaka Ranawaka said.
"America is trying to break up our country," said Labour Minister Gamini Lokuge.
The mass rally outside Colombo`s main railway station came hours after another protest march set off from outside the paramilitary Civil Defence Force (CDF) headquarters and headed for the nearby US embassy.
CDF personnel in civilian clothes took part in the march and were joined by others, witnesses said.
However, police anti-riot squads cordoned the area and allowed only a handful of protesters to carry a petition to the embassy.
Activists carried photographs of President Rajapakse and his defence secretary brother, Gotabhaya Rajapakse, who is credited with the strategy that crushed Tamil rebels in May 2009 after 37 years of fighting.
There was no immediate comment from the US embassy, but diplomatic sources said they had warned US nationals about the protests.
Police spokesman Ajith Rohana said demonstrators were peaceful.
President Mahinda Rajapakse`s legislator son Namal Rajapakse led protesters in their home constituency of Hambantota in the deep south of the island, the MP`s office said in a statement.
The US wants Sri Lanka to agree to a roadmap to probe war crimes allegations.
Sri Lanka has avoided censure at previous UN meetings thanks to backing from Russia and China. India, the island`s closest neighbour, has also supported Colombo.
Colombo admitted in a census report over the weekend that nearly 8,000 people were killed in the island`s north in 2009 when the final battle was fought -- less than the 40,000 deaths estimated by rights groups.
The UN estimates some 100,000 people died during Sri Lanka`s ethnic conflict between 1972 and 2009. (*)