A suspected Tamil Tiger suicide bomber blew herself up Sunday at the main railway station in Sri Lanka's capital, killing 11 people and wounding 92 others as the government prepared to celebrate the nation's 60th anniversary, military officials said.

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Sri Lankan police investigators search the site of an explosion at the railway station in Colombo, Sri Lanka, on Sunday. ((Eranga Jayawardena/Associated Press))

The attack in the heart of Colombo came amid a major increase in violence in this Indian Ocean island nation in recent months.

Government troops and separatist guerrillas have been battling in the jungles of the north, while suicide bombers and other militants have attacked civilians and military targets deep inside government-held territory.

Sunday's suicide attack was apparently aimed at civilians crowded into the Fort railway station in Colombo, a military spokesman said.

Many of the injured were taken to the National Hospital in the capital.

"Up to now, we have admitted 97 patients to the hospital, from the time this took place, about an hour ago, and out of them the doctors have confirmed that nine of them were dead," hospital director, Hector Weerasinghe told AP Television.

The explosion came a day after a bomb on a bus killed 18 people, mostly Buddhist pilgrims, in the central town of Dambulla, about 150 kilometres northeast of Colombo.

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A Sri Lankan police officer stands guard at the site of a bus explosion , in Dambulla, northeast of Colombo, Sri Lanka, on Saturday. ((Eranga Jayawardena/Associated Press))

Earlier on Sunday, a grenade exploded at a zoo on the outskirts of Colombo, wounding at least four people, the military said.

Renuka Bandaranayaka, assistant director of the zoo, said there had been many visitors at the time.

"I heard the bomb blast and I came to know it was a bomb and I evacuated all the visitors who were in at that moment and all my staff," she told AP television.

The attacks took place as the government prepared to hold celebrations across much of the country Monday to mark the 60th anniversary of Sri Lanka's independence from Britain.

The usually tight security across the capital was further heightened in recent days as troops searched cars and pedestrians ahead of a national ceremony on Monday morning on the city's major coastal road.

The Tamil Tiger rebels have been fighting since 1983 for an independent homeland for Sri Lanka's ethnic minority Tamils after decades of being marginalized by Sinhalese-dominated governments. The fighting has killed more than 70,000.

The military said Sunday that fighting between troops and rebels along the front lines surrounding rebel-held territory in northern Sri Lanka killed 32 rebels and two soldiers.

The rebels could not be reached for comment, but the two sides routinely give differing casualty figures.

More than 700 people have been killed in intensified violence since the government withdrew from a ceasefire with the separatist Tamil guerrillas last month.