Two police are reportedly among the dead.
Hundreds of people were wounded when soldiers and riot police used tear gas and armoured vehicles to drive out demonstrators from the square, the report said.
Protesters started gathering in Pearl Square about a month ago to demand political reform.
Around 1,000 security forces from Saudi Arabia and other Gulf of Arabia countries were sent to Bahrain on Monday. The United Arab Emirates also sent 500 police officers.
Members of Bahrain's Shia majority population have been engaged in protests against what they see as discriminatory policies by the tiny country's monarchy backed by the Sunni minority.
Bahrain has a population of around 800,000, and is home to the US Navy's Fifth Fleet.