The assailants, who arrived on a motorcycle, attacked the rally in the deep south district of Hambantota yesterday, the Marxist JVP or the People's Liberation Front said.
Two JVP activists were killed and several others injured in the attack. Troops have been deployed in Katuwana after the incident, military officials said.
Brigadier Ruwan Wanigasuriya the military spokesman said the deployment of the army was to assist police in maintaining law and order.
Police spokesman Superintendent Ajith Rohana said that some 22 people had been quizzed in connection with the incident.
The JVP legislator Anura Kumara Dissanayake charged a politically backed group of the government as being responsible. The government blamed the incident on JVP's factional rivalry.