At least two people have been killed and as many as 200 injured in clashes between anti-government demonstrators and security forces in Bahrain.
A doctor told the BBC he was treating many people with head and gunshot wounds, and that soldiers and police were using ambulances to attack people.
The violence came as the government announced a state of emergency and called in Saudi troops to keep order.
US Secretary of State Hillary Clinton has appealed for calm and restraint.
"The use of force and violence from any source will only worsen the situation," she told reporters during a visit to the Egyptian capital, Cairo.
"Our advice to all sides is that they must take steps now to negotiation towards a political resolution," she added.
Mrs Clinton also said she had told Saudi Foreign Minister Prince Saud al-Faisal that "they, along with everyone else, need to be promoting the dialogue".
A diplomatic row has also flared over the issue, with Bahrain recalling its ambassador in Tehran and complaining of "blatant interference" in its affair because Iran had condemned the arrival of foreign forces. BBC News
