Sri Lanka's military denied any involvement in the attack at around midnight on Thursday, at an office of the Tamil National Alliance (TNA) party outside the northern town of Jaffna.
Voters in northern Sri Lanka go to the polls today for a provincial election that threatens to stir up old animosity between the government and Tamils, four years after the military crushed separatist Tamil rebels and ended a 26-year war. Most voters in the north are Tamils and many of them are bitter about what they see as virtual occupation of the region by the army, which they accuse of rights abuses in the final, bloody stages of the war.
They are expected to vote overwhelmingly for the TNA, the former political proxy of the defeated rebels, in the election for 38 provincial councillors.
Kanakaratnam Sugash, the Jaffna district legal adviser for the People's Action for Free and Fair Elections (PAFFREL) poll monitoring group, said the attackers beat him with sticks and kicked him.
He said he was made to kneel and had a gun pointed at him: "They threatened to kill me," he said.