The Action Committee for Media Freedom, an umbrella organization of media rights groups, said police officers threatened the owner of the resort where the workshop was to be held in Negombo, a popular coastal town 38 km (24 miles) north of Colombo.
"A group of plainclothes police officers from the area and the intelligence unit warned the resort owner of dire consequences if they allowed the workshop to take place and the owner did not allow the workshop, fearing the repercussions," the group said in a statement.
Similar threats were made against three other gatherings earlier this year, the group said, adding to concerns over media freedoms in the Indian Ocean nation.
Sri Lankan government forces have been accused of widespread human rights violations in the final stages of its 26-year civil war which ended in May 2009. Colombo refuses to cooperate in a U.N. probe into the allegations, and in August denied visas to international investigators.
The Defence Ministry's NGO secretariat, which regulates non-governmental organisations, in July banned activist groups and NGOs from holding press conferences, issuing press releases and holding workshops for journalists.
Political analysts believe the government may be concerned that media workshops are connected to the U.N. probe.
In May, Sri Lanka blocked two news websites critical of the government, which press groups decried as intimidation.
Police denied making any threats in connection with the weekend's workshop, which was organised by the Professional Web Journalists Association.
"We don't have any involvement in this incident," national police spokesman Ajith Rohana told Reuters.
The owner of the resort, who did not want to be identified, declined comment but said the group's costs had been refunded.
© Thomson Reuters 2014