Sri Lankan police blocked hundreds of minority Tamils from attending a protest rally in the capital on Wednesday in a clampdown that was condemned by the embassy of the United States.
Buses carrying family members were prevented from leaving the northern town of Vavuniya to make a 260-kilometre (162-mile) journey to Colombo to protest against extra-judicial killings, said activist and politician Mano Ganeshan.
"The protest was called off because nearly 1,000 people were not allowed to proceed from Vavuniya," Ganeshan told AFP. "Armed police and army intimidated bus drivers and threatened them with dire consequences if they moved."
Military spokesman Brigadier Ruwan Wanigasooriya denied intimidation, but said police had "advised" bus operators against leaving in the interest of maintaining peace and calm in the area.
"The police stopped an organised movement of several buses in view of several complaints of stoning of buses in the area," Wanigasooriya said. "If these buses also proceeded, they could have been attacked and there could have been clashes."
In a statement headlined: "US alarmed by peaceful protestors' detention", the US embassy said "all Sri Lankans should enjoy the same rights and live in dignity".
It noted a "lack of progress" in ensuring basic human rights in Sri Lanka despite a UN Human Rights Council (UNHRC) resolution demanding an improvement.
"Since last year's UNHRC resolution, the US has grown increasingly concerned by the lack of progress on these issues, as well as backsliding on other important areas of fundamental democratic rights," the statement said.
Minority Tamils say thousands are still missing nearly four years after troops crushed Tamil Tiger rebels in May 2009 and ended 37 years of ethnic bloodshed in the majority Sinhalese nation.
The latest confrontation between Tamils and police came as Sri Lanka faced fresh censure at the ongoing UNHRC sessions in Geneva.
Meanwhile, a previously unknown group calling itself the "Dead and Missing Persons Parents Front" said it petitioned the UN to investigate war crimes by the defeated Tigers.
Rights groups have said that up to 40,000 ethnic Tamils were killed in the army's final battle against Tamil rebels while Colombo maintains that not a single civilian was killed by them.