The United States led Western criticism of Sri Lanka on Tuesday after pro-government activists broke up a meeting of US diplomats and ethnic Tamils who lost loved ones in the island's separatist war.
The US embassy said activists led by Buddhist monks stormed the meeting in a Catholic Church compound in Colombo on Monday and "behaved threateningly towards the families".
The embassy said police appeared to support the mob, which halted the meeting of foreign diplomats, including from the US, and families whose relatives went missing during the conflict that ended in 2009.
"The United States strongly urges the government of Sri Lanka to enforce the rule of law and permit all citizens to exercise their most basic human rights, including freedom of speech," the embassy said in a statement.
"We also call on the government of Sri Lanka to take all possible steps to ensure the safety of families who had travelled from the north (the former war zone) to attend this meeting, both in Colombo and upon their return home," the embassy added.
Critics have accused President Mahinda Rajapakse's government of a renewed attempt to crack down on dissent, after it last month banned NGOs from holding press conferences on everything from exposing corruption to promoting voters' rights.
France, Germany, Britain and Switzerland joined the US in demanding that the government respect its own laws guaranteeing freedom of association.
"We strongly urge the government to ensure and respect freedom of assembly and expression in Sri Lanka," the four Western nations said in a joint statement issued by their embassies in Colombo.
A group of Sri Lankan lawyers criticised the apparent intimidation of the families and demanded police prosecute those responsible.
Disruption of the meeting follows recent incidents in which pro-government mobs have broken up US-funded workshops for journalists from Sri Lanka's ethnic Tamil minority.
These incidents also sparked criticism from the US embassy which urged Sri Lanka to crack down on harassment of journalists and to protect their rights.
The country's main media rights group, the Free Media Movement, said dozens of pro-government activists blocked Tamil journalists from holding one of the training sessions in Colombo late last month.
Colombo faces an international investigation over allegations that government forces killed at least 40,000 Tamil civilians in the final months of the war between the military and rebels fighting for a separate homeland for Tamils.
Colombo has long denied the charge.
The murders of more than a dozen journalists and media workers during the decades-long war have remained unsolved for the past 25 years.