Brazilian police have seized 40 tons of marijuana, 150 kilograms of cocaine and a large amount of weapons and ammunition in an action against drug gangs in one of major drug traffickers’ strongholds in Rio de Janeiro, police officials said.

Some 2,600 paratroopers, marines and elite police backed up by helicopters and armoured personnel carriers led a pre-dawn assault on Sunday on the traffickers' bastion of Grota, a lawless city within a city with a population of 400,000, BBC reported.

According to the police statement, the seized marijuana, packed for distribution, was found in several houses in Grota favela.

The criminals have been given an ultimatum to surrender until sunset on Saturday, or face an assault.