FMM's Jayasekera said he also received death threats over the telephone for holding a press conference to denounce the latest harassment of Tamil journalists.
The calls originated from numbers that cannot be traced, he added.
Murders of more than a dozen journalists and media workers have remained unsolved for the past 25 years.
Journalists from the northern Jaffna peninsula, a former war zone, had travelled to Colombo for an electronic media workshop.
"The organisers were forced to stop the program because of intimidation by dozens of protesters," said Sunil Jayasekera, spokesman for the Free Media Movement (FMM), the country's main media rights organisation.
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"We were concerned about the safety of the journalists and that is why the training was called off."
"They carried placards saying that we are supporting (separatist) Tamil Tigers (guerrillas)," Jayasekera told reporters.
The New York-based Committee to Protect Journalists (CPJ) has accused Sri Lanka of keeping up a p