Bahrainis wielding knives and sticks attacked Shia villagers overnight, witnesses have said, as pressure grew for this month's Formula One race in the Sunni-ruled state to be cancelled over the unrest.
A day after a gasoline pipe bomb injured seven policemen, men from Sunni neighbourhoods who answered an internet call to avenge the attack converged on Shia-populated villages near Manama late on Tuesday, according to witnesses.
"I saw hundreds of men carrying knives, sticks and other sharp objects," on the outskirts of several Shia villages, said one man who identified himself only as Hussein.
"They were stopping cars and asking passengers where they lived in order to determine what sect they belonged to," he said by telephone, referring to the Sunni and Shia communities in Bahrain.
Al-Wefaq, the Gulf kingdom's largest Shia opposition group, said the attackers were in civilian clothes and "beat up" the residents of Shia villages.
A statement by the group accused security forces of failing to stop the attacks.
"The security forces did not carry out their duty. They did not disperse the [assailants] or prevent them from attacking citizens," the statement said, adding that the authorities must "deal with these militias".
But Bahrain's interior ministry said police "prevented" a group of unidentified assailants from entering the Shia village of Al-Nuweidrat on the outskirts of the capital.
In a statement, the ministry said assailants "attacked 24-hour shops and destroyed two cars," after holding an "illegal gathering," without mentioning the reported attacks by Sunnis on Shia neighbourhoods.
Witnesses also said mobs attacks the headquarters of a major Shia owned business chain whose holdings include supermarkets and cafes.
Amir Jawad, a board member for the Jawad Business Group, said the mob smashed windows and caused other damage to the supermarket in what he called a "systematic" series of attacks.
The crowds also roamed outside the headquarters of the company, which owns supermarkets, cafes and fast-food outlets.
Jawad said company security guards detained at least two suspected attackers. There were no immediate reports of injuries.
"The vigilantes used iron, steel and wood sticks," he said.
Jawad sites have faced sporadic vandalism since the uprising began in February 2011 in the gepgraphically strategic nation, which is home to the US Navy's 5th Fleet.
Concern over Formula One
The growing unrest could escalate worries by Formula One teams about whether to participate in the April 22 Bahrain Grand Prix.
The youth group of the "Revolution of February 14" has called for "three days of rage" in Bahrain from April 20 to 22, and also launched a campaign on Twitter to cancel the race.
On Tuesday, F1 supremo Bernie Ecclestone admitted this week that he was unable to force teams, reluctant to race in Bahrain over security concerns, to participate in the event.
A government statement released on Wednesday assured visitors the race would be held in "safe and stable conditions," adding the bouts of unrest are "infrequent" and happen "in distant villages".
Last year's Grand Prix event was initially postponed and later cancelled following the February and March protests inspired by Arab uprisings.
John Yates, a former assistant commissioner in the London Metropolitan Police Service who is an adviser to Bahrain's interior ministry on policing, has said there "is nothing that in any way warrants for the race to be postponed".