Riot police in Bahrain have launched an assault on anti-government demonstrators camping in a square in the capital, Manama.
The police fired tear gas at the mainly Shia demonstrators, who have been calling for political reform from the country's Sunni rulers.
The goverment has declared martial law and called in Saudi troops to help keep order.
At least two people died in clashes on Tuesday.
It is not clear whether forces from other Gulf states are taking part in the crackdown.
The BBC's Caroline Hawley in Manama says black plumes of smoke are rising over Pearl Square, the centre of the protests.
Armoured vehicles moved in shortly after dawn, our correspondent says, and the security forces now appear to have full control of the square.
Helicopters flew overhead
Bahrain - home to the US Navy's Fifth Fleet - is the first Gulf country to be thrown into turmoil by the wave of unrest sweeping the Arab world.
The protests in Bahrain - which has a population of 525,000 - began last month. The Shia majority complain of economic hardship, lack of political freedom and discrimination in jobs in favour of Sunnis.
The king reshuffled his cabinet on 26 February, replacing four ministers, including two royal family members. However, he did not sack Prime Minister Sheikh Khalifah ibn Salman al-Khalifah, who has held the office for more than 40 years.