MANAMA, Bahrain—Bahraini opposition groups claimed Saturday that a man was killed during clashes with security forces, threatening to sharply escalate the Gulf nation's unrest as officials struggle under the world spotlight as hosts of the Formula One Grand Prix. Authorities opened an investigation in a bid to defuse tensions.

At least 50 people have died in the unrest since February 2011 in the longest-running street battles of the Arab Spring. Bahrain's Shiite majority seeks to break the near monopoly on power by the ruling Sunni dynasty, which has close ties to the West.

Persistent protests have left the country's rulers struggling to keep attention on the buildup to Sunday's Formula One race—Bahrain's premier international event. It was called off last year amid security fears and Bahrain's leaders lobbied hard to hold this year's event in efforts to portray stability and mend the country's international image.

A statement by Bahrain's Interior Ministry said a probe was under way, but gave no other immediate details. The death, however, was likely to intensify a wave of expected protests to coincide with the Formula One.

The body was found in an area west of the capital Manama, where clashes broke out after a massive protest march Friday. Opposition factions said riot police and demonstrators were engaged in running skirmishes around the village of Shakhura, about five miles west of the capital Manama and known for its burial mounds dating back more than 5,000 years.

The area is 12 miles from the Grand Prix circuit, which is surrounded by desert and under round-the-clock security watch.

The Interior Ministry said the man who died was identified as Salah Abbas Habib Musa, 36, and the case was "being treated as a homicide." The ministry statement didn't give a cause of death but said investigators found "a wound" on the left side of his body.

Social media sites have urged more demonstrations as part of the opposition's effort to use the world attention from the race to press their demands for a greater political voice.

Sporadic clashes broke out Saturday in Shiite villages west of Manama, where plumes of black smoke rose from tire blockades set ablaze by protesters. On the main highway from Manama—used by many F1 teams and fans—work crews tore down protest banners with images of people killed during the uprising.

"We demand democracy," and, "Down, Down Hamad," chanted some of the tens of thousands of opposition supporters Friday in reference to King Hamad bin Isa Al Khalifa, as they massed on the main highway leading out of Manama. Bahrain's monarchy is the main backer of the F1 race, and the crown prince owns the rights to the event.

Hours before the march, Bahrain's most senior Shiite cleric, Sheik Isa Qassim, delivered a strongly worded sermon that denounced authorities for making dozens of arrests of suspected dissidents in recent weeks. He said the intensified crackdowns before the F1 event were "as if we are entering a war" in the strategic kingdom, which is home to the U.S. Navy's 5th Fleet.

Shiites account for about 70 percent of Bahrain's population of just over half a million people, but claim they face widespread discrimination and lack opportunities granted to the Sunni minority. The country's leaders have offered some reforms, but the opposition says they fall short of Shiite demands for a greater voice in the country's affairs and an elected government.

The unrest has put Washington into an awkward position. U.S. officials have called for efforts to reopen political dialogue in Bahrain, but are careful not to press too hard against the nation's leadership and possibly jeopardize its important military ties.

In Washington on Friday, State Department spokeswoman Victoria Nuland expressed the Obama administration's concern about "the increase in violence in Bahrain, especially leading up to the Formula One race."

"These are unproductive, unhelpful acts in building the kind of meaningful trust and reconciliation that is needed in Bahrain," she told reporters. "We're calling for, again, Bahraini government respect for universal human rights and demonstrators' restraint in ensuring that they are peaceful."

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