MANAMA, Bahrain - Mobs with iron rods and sticks ransacked a supermarket belonging to a major Shiite-owned business group Wednesday, a company official said, as part of a spike in violence in the Gulf nation in retaliation for a bomb attack on police.

The attack appeared linked to a wave of reprisals and intimidation by suspected Sunni groups angered by the 14-month-old uprising by Bahrain's Shiite majority seeking to weaken the powers of the kingdom's Sunni monarchy.

The growing unrest, which has included vigilante-style attacks in some Shiite areas, also could escalate worries by Formula One teams about whether to participate in the April 22 Bahrain Grand Prix. The race was called off last year amid security fears and Bahrain's leaders are pushing hard to bring back the event as a sign of stability in the island nation.

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 strategic Gulf nation, which is home to the U.S. Navy's 5th Fleet. Bahrain's Shiites account for about 70 per cent of the population, but they claim they face widespread discrimination and are kept from top government or military posts.

Nearly 50 people have died in the Arab Spring's longest-running street battles. Apparent Sunni mobs have stepped up reprisals following a blast Monday that injured seven policemen. On Tuesday, Bahrain said four suspects had been arrested in connection with the blast.

In some Shiite areas, apparent Sunni hard-liners carrying knives and sticks staged hit-and-run attacks late Tuesday and set up roadblocks, said witnesses, who spoke on condition of anonymity in fear of receiving pressures from authorities.

The largest Shiite political group, Al Wefaq, claimed that security forces failed to confront the "militias."

In Egypt, a prominent Bahraini activist was barred Wednesday from entering the country upon the request of a security agency, an airport official said.

Nabeel Rajab is the second Bahraini activist to have difficulties coming into Egypt in recent weeks. Pro-democracy groups say authorities have been cracking down on networking between regional activists.

Activists say the deportations indicate that while authoritarian president Hosni Mubarak may have been removed during last year's uprising, his security agencies are unchanged and may be tracking dissenters even more closely than before.

Rajab wrote on his Twitter account before leaving: "In Mubarak's Egypt, we were never barred from entering. Now in post-revolution Egypt, we are."