The blast came a day after a speech by Tiger leader Velupillai Prabhakaran in which he warned that they would hit back at the government.
The attack was in front of the Eelam People's Democratic Party (EPDP) office, on Park Road in the Narahenpita area of Colombo city limits, the Defence Ministry said.
EPDP leader Douglas Devananda, a former Tamil militant, has survived several attempt on his life by the Tigers.
At least three people were wounded in the blast, the AFP news agency quoted police as saying.
Three injured, two in serious condition, were brought to the city's General Hospital, a hospital official told AFP.
Police and the military said they were still checking on casualties and whether the bomber was dead.
(Updated)