Lebanese security forces stormed a unit of the country's largest prison on Monday where Islamist militants are detained, security sources said, as authorities searched for those responsible for a double suicide attack at the weekend.
Inmates torched mattresses to protest the move, the sources said, but there were no injuries. The sources said the police were searching for "forbidden items" at Building B in Roumieh prison, where Islamist militants are held.
Roumieh was originally built to hold about 1,500 inmates, but is now crammed with about 3,700.
The prisoners are able to communicate outside with mobile phones and security sources have said some are still involved in attacks against the state.
Television footage showed police entering and searching prisoners' rooms and smoke rising from the prison.
A double suicide attack on Saturday killed eight people at a cafe in the Lebanese city of Tripoli. It was claimed by the al Qaeda-linked Nusra Front, members of which live in Roumieh.