The demonstration in Colombo demanded a halt to violence, threats and intimidation after Thursday's shooting of Lasantha Wickrematunga, chief editor of the Sunday Leader, which has been highly critical of the government.
Opposition lawmakers also protested inside parliament, accusing the government of being responsibile for the killing.
The New York-based Committee to Protect Journalists (CPJ) asked ambassadors in Colombo to "weigh in forcefully and immediately" with President Mahinda Rajapakse to put an end to the attacks "raining down" on Sri Lanka's media.
"The international community in Colombo must act quickly to bring pressure on President Rajapakse to reverse this murderous trend," the CPJ said.
It said the sheer brutality of the attacks in recent days was a clear indication of how the war on the Sri Lankan media had moved far beyond the use of threats, intimidation, legal harassment, and sporadic violence.
The United States, European Union, India and the World Bank also j