The US embassy said activists led by Buddhist monks stormed the meeting in Colombo on Monday and "behaved threateningly towards the families of the disappeared".
The embassy said the police appeared to support the mob, which successfully halted the meeting with families whose relatives disappeared or were killed during Sri Lanka's drawn-out separatist war that ended in May 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 and freedom of assembly," 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.
The local organisers of the meeting said the pro-government mob used threatening language against diplomats too.
Disruption of the meeting follows recent break-ups by pro-government mobs of 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.