[WARNING: This video contains images that some viewers may find disturbing] Troops and tanks have locked down the Bahraini capital of Manama after riot police swinging clubs and firing tear gas smashed into demonstrators in a pre-dawn assault, killing at least four people. Hours after the attack on Manama's main Pearl Roundabout, the military announced a ban on gatherings, saying on state TV that it had "key parts" of the capital under its control. Khalid Al Khalifa, Bahrain's foreign minister, justified the crackdown as necessary because the demonstrators were "polarising the country" and pushing it to the "brink of the sectarian abyss". Speaking to reporters after meeting with his Gulf counterparts, he also said the violence was "regrettable". Two people had died in police firing on the protesters prior to Thursday's deadly police raid. An Al Jazeera correspondent, who cannot be named for security reasons, went to Salmaniya hospital, which was thrown into chaos by a stream of wounded protesters from Pearl Roundabout.