The extent of Kumaratunga's injury was not immediately clear. Witnesses said the explosion took place as she was about to get into her car after attending the meeting outside the town hall in downtown Colombo.
"She was waving at the crowd as she was getting inside the car when I heard a big bang and after that there was total confusion," said a witness who asked not to be named.
Security commandoes fired immediately and some journalists said at least one photographer was injured.
"There is total confusion. We are trying to figure out what really happened," a senior police official said on condition of anonymity.
Kumaratunga, of the People's Alliance party, is running for a second term in office, seeking a renewed mandate to end the country's 16-year ethnic war. She is pitted against Ranil Wickremesinghe of the opposition United National Party and 11 other candidates in the Dec. 21 election.
No one immediately claimed responsibility for the blast, but suspicion fell on the separatist Tamil Tiger rebels, who are fighting for a homeland.
The Tigers have been blamed for several assassinations and bomb blasts in Sri Lanka over the years.
The bombing came a day after Sri Lankan troops fought back an assault by rebels in boats on a strategic military base in the north, leaving 128 rebels and soldiers dead, the Defense Ministry said Saturday.
Dozens of rebel boats tried Friday to cross a lagoon south of the Elephant Pass military complex, where fighting has raged for more than a week.
Soldiers destroyed one large boat carrying guerillas and badly damaged four others trying to cross the lagoon, the Ministry said in a statement. Troops later also fought back another assault by rebels on speed boats.
The attack was led by the Sea Tigers, the naval unit of the rebels.
Air force helicopter gunships destroyed a few more boats, the statement said. Military officials confirmed that rebels earlier shot down an air force MI-24 helicopter over the Kilalli lagoon, killing all four crew members.
Rebel communications monitored by the military said 110 guerillas were killed in Friday's battle. The ministry said 18 soldiers were killed and 57 were wounded.
The military says 590 rebels and 46 soldiers have been killed and 397 soldiers wounded since fighting erupted near the Elephant Pass. The rebels say only 78 of their fighters 31 of them women have been killed. Independent confirmation was not possible as reporters are not allowed into the war zone.
The rebel Voice of Tiger clandestine radio, monitored in the northern garrison town of Vavuniya, said the guerrillas were 3.5 miles from the heavily fortified military complex.
Guerillas were fighting their way toward Elephant Pass after Friday's capture of a major army camp south of the base, the radio said. Hundreds f soldiers were killed when rebels overran the Paranthan camp, which forms part of the Elephant Pass defenses, the radio said.
Saturday's military statement made no reference to Paranthan, and officials were not immediately available for comment on the rebel claim.
Last weekend, the guerillas made several attempts to breach the defenses of Elephant Pass, which guards the causeway linking the northern peninsula with the rest of the country.
Capture of the Elephant Pass would provide the rebels with a platform to carry out attacks on Jaffna, their former stronghold, 185 miles north of capital Colombo.
Jaffna was seized by the army in 1996, which was a severe blow to the Tigers who want to carve out a Tamil homeland in the north and the east.
The rebels accuse the Sinhalese majority of discriminating against Tamils in jobs and education. An estimated 61,000 people have been killed since the war began in 1983.