Bahrain protests in Manama's Pearl Square overrun
Two people are reported to have died after police fired tear gas 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.
It is not clear whether soldiers from other Gulf states are taking part in the crackdown alongside Bahraini forces.
The BBC's Caroline Hawley in Manama says black plumes of smoke are rising over Pearl Square, the centre of the protests, while helicopters are flying overhead. There are riot police stationed in approach roads.
Armoured vehicles moved in shortly after dawn, our correspondent says, and the security forces now appear to have full control of the square.
Hospital surrounded
The crackdown comes a day after King Hamad Bin Isa Al Khalifa declared a three-month state of emergency.
Protesters had set up barricades in the square but they were no match for the military, our correspondent says.
Tyres were burnt to alert people in surrounding villages to the crackdown but they were unable to make it through to the square to join the protests.
The security forces have now moved into Manama's financial district, reopening roads which had been blocked by protesters; Kazinform cites BBC News.
See www.bbc.co.uk for full version