The call was made a day after the UNP's T. Maheswaran was gunned down by unknown gunmen at a Hindu temple in a suburb north of the capital Colombo Tuesday.
Maheswaran has survived an earlier assassination attempt by Tamil Tigers, but he has recently been critical of the government, which the UNP said should bear responsibility for his death.
Lawmaker Mangala Samaraweera, a former government minister who defected to the opposition, told reporters the government is operating 'goon squads'.
"There is obviously a culture of impunity prevailing in this country and the people must say enough is enough and ask for the truth."
Human rights groups have accused government forces of human rights abuses after scores of people were abducted or killed in recent months.
The government has denie