Security forces with tanks have overrun a square in the centre of Bahrain's capital, Manama, where anti-government protesters have been camped for weeks.
Two people are reported to have died after police fired tear-gas and possibly live rounds at the mainly Shia demonstrators, who have been calling for political reform.
The country's Sunni rulers have declared martial law and called in Saudi troops to help keep order.
At least two people died in clashes on Tuesday and more than 200 were injured.
The BBC reports it's not clear whether soldiers from other Gulf states are taking part in the crackdown alongside Bahraini forces.
The BBC's correspondent in Manama says black plumes of smoke are rising over Pearl Square, the centre of the protests, while helicopters are flying overhead.
Armoured vehicles moved in shortly after dawn and the security forces now appear to have full control of the square.
The crackdown comes a day after King Hamad bin Isa al Khalifa declared a three-month state of emergency.
The protests began last month. The Shia majority complain of economic hardship, lack of political freedom and discrimination in jobs in favour of Sunnis.