Ranjith Gunasekara, a police spokesman, said the men opened fire and hurled hand grenades - forcing the network's three TV stations and four radio channels off air for several hours.
There were no reports of injuries in the attack.
Firefighters battled the resulting blaze, as an army bomb squad was brought in to comb the building for more explosives.
The broadcaster has been criticised by state media for devoting too much airtime to a suicide attack by a Tamil Tiger separatist last week.
Separatist campaign
The police and the government have also clashed with the station over its coverage of the conflict between Sri Lankan troops and Tiger separatists.
Meanwhile, concern over the number of civilian deaths are mounting as Sri Lankan troops engage in heavy fighting in a bid to capture Tiger strongholds in the north of the island.
The government announced on Monday that it had taken the rebel-held town of Oddusudan and chased Tiger fighters out of their de facto state headquarters in the north.
The army also said it had seized control of the key Elephant Pass which links the northern Jaffna peninsula to the mainland.
However, human rights activists have warned that the 300,000 civilians living in separatist-held territory risk being caught in crossfire.
Keheliya Rambuikwella, a government defence spokesman, said troops were "taking the utmost precautionary measures", but declined to outline what those precautions were.
Last month, Human Rights Watch accused rebel fighters of preventing civilians from fleeing their territory. The group also alleged the government had arbitrarily detained and mistreated the estimated 1,000 civilians who have fled across the front lines.
It is difficult to verify battle accounts and casualties as reporters are barred from the war zone.
The rebel Tigers have waged a long war for independence for Sri Lanka's minority Tamil community, who have been marginalised by successive governments controlled by the Sinhalese majority.
About 70,000 people are thought to have been killed since separatist fighting began in 1972.