MANAMA - Bahraini forces backed by helicopters wednesday launched a crackdown on protesters imposing a curfew and clearing hundreds from a camp that had become the symbol of an uprising by the Shi'ite Muslim majority Hospital sources said three policemen and three protesters were killed in the assault that began a day after Bahrain declared martial law to quell sectarian unrest that has sucked in troops from fellow Sunni-ruled neighbour Saudi Arabia. A protest called by the youth movement, which had been leading protests at the Pearl roundabout, could not be held, after the military banned all marches and gatherings and imposed a curfew from 4 pm to 4 am across a large swathe of Manama. Mansoura Al Jamri, a journalist at Bahrain's only independent Al-Wasat newspaper, said tanks closed the area where the protest was planned to be staged. "Tanks managed by the Bahraini National Guard and some other forces from Saudi Arabia and Emirates cordoned the area of Budaya and banned protesters from reaching it," Mansoura told The Gazette in an e-mail interview. She added that by midday all local and international calls were prohibited. "I do expect an Internet outage would follow," she added. Al Jamri, a Shi'ite whose father was a leader of Bahraini opposition, said that hundreds of foreigners were planning to leave Bahrain due to the reigning instability. Gulf Arab ruling families are Sunni, and analysts say the intervention of their forces in Bahrain might provoke a response from Iran, which supports Shi'ite groups in Iraq and Lebanon. Iranian President Mahmoud Ahmadinejad said Bahrain's crackdown was "unjustifiable and irreparable". "Today, we witness the degree of pressure imposed on the majority of people in Bahrain," he said according to official TV.