Soldiers fired tear gas and shot heavy weapons into the air as thousands of protestors in Bahrain have defied a government ban.
Hospital officials said at least five people were killed and 20 seriously injured as protestors marched toward the Pearl Square in the capital Manama.
Unconfirmed reports from the Associated Press said security forces had fired tear gas at protesters.
British Foreign Secretary William Hague has warned against all but essential travel to the island nation.
He said: “We have conveyed our concern about these events and the level of violence to the Government of Bahrain.”
“We are advising all British nationals to stay away from protests and to avoid all but essential travel around Bahrain. The airport in Manama continues to function normally, but we will of course keep this situation under review and ensure that British nationals in Bahrain receive full consular support.”
The clashes came just hours after worshippers at Friday prayers called for the toppling of the US-allied monarchy in Bahrain, which is home to the US Navy’s 5th Fleet.
The mood has turned more defiant against the entire ruling system after the crackdown on a protest encampment in Manama, which left at least five dead, and more than 230 injured.
Pro-government supporters have also held a rally in Manama.
After prayers in the Grand Mosque, demonstrators with Bahraini flags and portraits of the king staged their own march protected by security forces.
The rally has attracted many non-native Bahrainis, including Sunni Muslims from other Gulf states and South Asia.
The White House has expressed “strong displeasure” about the rising tensions in Bahrain. The 5th Fleet is the centrepiece of the Pentagon’s efforts to confront growing Iranian military ambitions in the region.
The pro-government gathering had many non-native Bahrainis, including South Asians and Sunni Arabs from around the region.
In Manama, soldiers guarded the capital’s main areas and placed roadblocks and barbed wire around Pearl Square and other potential gathering sites.
On Thursday, Bahrain’s leaders banned public gatherings to try to keep the protest movement from re-igniting. But the underlying tensions in Bahrain run even deeper than the rebellions for democracy that led to regimes stepping down in Tunisia and Egypt.
The Bahrain violence forced the cancellation of a motor race in Bahrain over the weekend, and leaves the March 13 season-opening Formula One race at the same track in doubt.
Formula One chief Bernie Ecclestone said he will decide next week whether to proceed with the race.