Authorities in Bahrain opened an investigation Saturday after the body of a man was found near the scene of clashes between protesters and security forces.

Opposition groups claimed the man was killed by riot police in another possible blow to the Gulf nation as it struggles to quell unrest during the highly awaited return of the Formula One Grand Prix car race.

A statement by Bahrain's Interior Ministry said a probe was under way. The death nonetheless will likely intensify a wave of expected protests against the race.

On Saturday, confrontations between police and anti-government protesters turned violent in the Shia opposition stronghold of Diraz. Opposition supporters, numbering about 150, set tires ablaze and riot police fired dozens of rounds of tear gas to disperse them. No injuries were reported.

"Down, Down Hamad" and "We don't want Formula One," the protesters shouted in reference to King Hamad bin Isa Al Khalifa. 

The heavily guarded Bahrain International Circuit, where the F1 is taking place, is about 15 kilometres away from the demonstration.

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People in uniforms stand next to a body found on the roof of a small structure west of the Bahraini capital. (Bahrain Media Center/Associated Press)

Race drivers have mostly kept quiet about the controversy surrounding the Bahrain GP.

Asked about Musa's death after taking pole position in Saturday's qualifier, F1 world champion Sebastian Vettel said, "I think it's always dreadful if someone dies."

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

The body was found in an area west of the capital Manama, where clashes broke out after a massive protest march Friday.

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

The protests have left the country's rulers struggling to keep attention on the buildup to the 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.

But protesters also have seized on the worldwide attention since race teams and fans began arriving this week.

"We demand democracy" 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.

Unrest puts Washington in awkward position

Hours before the march, Bahrain's most senior Shia cleric, Sheik Isa Qassim, delivered a strongly worded sermon that denounced authorities for making dozens of arrests of suspected dissidents in recent weeks. He called 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.

Hunger strike continues

Eight prominent opposition figures in Bahrain have been sentenced to life in prison on charges of trying to overthrow the state, including rights activist Abdulhadi al-Khawaja, whose two-month and counting hunger strike has galvanized the Shia resistance.

In addition to refusing food for more than two months, al-Khawaja's wife said her husband began refusing IV drips and water this week.

"I am really worried about him," Khadija al-Musawi told the Associated Press in a phone interview Wednesday. "The government wants my husband dead."

Shia account for about 70 per cent 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 Shia 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 1 race."

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

Clashes take place nearly every day with demonstrators hurling firebombs and riot police responding with tear gas and sometimes firing birdshot. The main Shia political group, Al Wefaq, says at least 50 people have been injured in the past two days when security forces fired pellets to disperse protesters.

The rulers have depicted the race — expected to draw a worldwide TV audience of about 100 million in 187 countries — as an event that will put the divided society on the path of reconciliation.