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.
Earlier, Bahrain's King Hamad Bin Isa Al Khalifa decreed that there would be a three-month state of emergency to help restore order.
The head of the armed forces had been authorised to take all measures to "protect the safety of the country and its citizens", the announcement said.
On Monday, more than 1,000 troops from the Gulf Co-operation Council rolled into the country at the king's request, flashing victory signs.
Thousands of Bahrainis marched on the Saudi embassy in the capital, Manama, on Tuesday to protest against the intervention.
"People are angry. We want this occupation to end. We don't want anybody to help the Al Khalifa or us," a protester called Salman told the Reuters news agency, referring to the Sunni Muslim ruling family.
After the declaration of the state of emergency, many protesters set up barricades to protect themselves against the security forces. Young men, some wearing masks and carrying sticks, stood guard.
Later, there were violent clashes in several mainly Shia areas. In the village of Sitra, 15km (10 miles) south of Manama, police officers armed with shotguns fired on residents, a witness told the BBC.
Medics said more than 200 people had been injured in the clashes, and that two men - one Bahraini and the other Bangladeshi - had been killed.
State television said a Bahraini policeman was among the dead, denying media reports that a Saudi soldier had been shot and killed.
There were chaotic scenes at the Salmaniya medical centre, where many of the injured were brought.
A doctor at the medical centre's accident and emergency department said there were "many, many casualties".
"People are coming in with bullet wounds and injuries caused by rubber bullets. There are hundreds of people," he told the BBC. "We received one major case - a man whose skull had been split open by something."
Two other men were in a serious condition after being shot in the eyes, while a third had been shot in the back of the head, the doctor said.
"We were at the health centre in Sitra, and they shot at us. The doctors and nurses were all scared because the windows were being broken and we could hear the shooting. This is a disaster," he added.
He said police and soldiers - both Bahraini and foreign - had seized six ambulances, and then used them to attack protesters.
"The paramedics were kicked out, and they took the ambulances. They went everywhere in them and they were shooting people."
Other doctors appealed for international help to ensure access to the wounded. The BBC saw an ambulance that had been shot at.
The main Shia Muslim opposition group, al-Wefaq, condemned the state of emergency, and appealed for international help.
On Monday, it said the arrival of Gulf states troops - the first time that any Arab government has called for outside military help during the current wave of protests sweeping the region - was tantamount to a declaration of war.
Most members of Bahrain's majority Shia community majority community, which has long complained of discrimination and repression by the Sunni elite, say they want a constitutional monarchy and other democratic reforms. However, some have said they want a republic.