COLOMBO - Sri Lanka marked victory over Tamil Tiger rebels with a national holiday on Wednesday, though security forces were on a state of high alert against revenge attacks.

The holiday was to celebrate the military defeat of the Tigers after decades of bloodshed on the island, a government official said.

'The government has also asked all state institutions to fly the national flag for one week,' he said.

Defence spokesman Keheliya Rambukwella said security levels would remain high to guard against attacks from any remaining Tigers, whose leader was shot dead by troops on Monday.

Velupillai Prabhakaran's body was shown on national television and President Mahinda Rajapakse declared in parliament that the conflict had ended.

The government's triumph came at the cost of many innocent lives, according to the United Nations.

The UN and human rights groups have blamed indiscriminate shelling by the military for causing many civilian casualties, while accusing the rebels of using tens of thousands of people as a 'human shield'.

Relief agencies have complained that access to government-run camps housing tens of thousands of displaced civilians has been further restricted in recent days.

UN Secretary General Ban Ki-moon is to visit Sri Lanka on Friday and Saturday to push for reconciliation on the island.