Bahrain's Formula One grand prix will go ahead despite a growing international outcry about the staging of the race in the Gulf state that intensified on Saturday following the discovery of the body of a protester allegedly abducted from a village by security forces.

According to the opposition party Wefaq, the body of 36-year-old Salah Abbas Habib Musa, a father of five, was found on a rooftop in the Shia village of Shakhoura the day before the race.

With dozens of armoured personnel carriers guarding the main route to the circuit, the decision by F1 and the Sunni minority royal family to push ahead with the event – partly to help convince the world of Bahrain's return to normality – appeared to be degenerating into a human rights and PR catastrophe.

Despite claims by F1 chief Bernie Ecclestone and regime officials that the race was safe and the threat of violence "hyped", the buildup to the contest has been marked by increasingly large anti-government demonstrations that have been put down with teargas, birdshot and stun grenades.

On Friday, activists began what they described as the first of "three days of rage" against Bahrain's rulers. There were reports last night that police firing teargas canisters were confronting protesters hurling petrol bombs.

The ruling al-Khalifa family has depicted the race — which is expected to draw an audience of about 100 million — as a "force for good" and an event that will unite Bahrain.

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 is seeking to break the near-monopoly on power by the ruling Sunni dynasty, which has close ties to the west and Saudi Arabia.

Wefaq official Sayed Hadi al-Mousawi said it was not clear what caused Habib's death. "We haven't got the body because the official investigators have surrounded the area, but we understand he was beaten severely. His colleagues with him last night were beaten with batons and the butts of rifles used to shoot teargas and birdshot." Bahrain's interior ministry described Habib as having suffered "a wound to his left side".

Wefaq, the leading party among Bahrain's restive Shia population, published a photograph of Habib's body splayed on a corrugated-iron rooftop.

According to activists, Habib had been among a small group protesting in the village of Abusaiba on Friday night when it was stormed by security forces. According to some reports, he was one of three people injured.

Images posted on Twitter on Saturday afternoon of the other men detained with Habib appeared to show they had all been severely beaten, with their bodies showing large identical weals.

Pictures of Habib's body shown on Bahraini television appeared different to a long-lens shot of his body taken from a neighbouring roof just after the body's discovery, suggesting that sticks had been placed around his body and a catapult placed in his hand.

Told of the death, F1 world champion Sebastian Vettel, who had just qualified in pole position for the race, said: "I think it's always dreadful if someone dies, but I don't know what happened."

The decision to go ahead with the race was defended by Jean Todt, president of the FIA, the sport's governing body, who echoed Ecclestone's comments late last week criticising the reporting of the situation in Bahrain. "I came here after the Indian Grand Prix to assess the situation and to understand better the situation. I had discussions with the British ambassador, the French ambassador, the Italian ambassador, the German ambassador – and the authorities," he said. "Everybody was very comfortable with the situation and about the implementation of new solutions for the country."

At a press conference held just before the qualifying rounds of the race, opposition figures accused the regime of using the worst violence against protesters when there were no foreign witnesses. Khalil Marzooq, one of 18 Wefaq MPs to stand down during the violent suppression of last year's anti-government protests, also expressed scepticism over the willingness of officials to investigate Habib's death – accusing Bahrain of a "chronic lack" of independence in police inquiries. He accused security forces of allowing gangs of pro-regime "thugs", armed with nailed clubs and iron bars, to operate with impunity.

The royal family has paid millions for the rights to hold a race on the island and had been counting on the running of this year's race to signal to the world a return to normality.

The government initially crushed the protests last year by introducing martial law and bringing in Saudi troops. But opposition parties resumed organising street rallies, and youths in mainly Shia villages have clashed with riot police daily.

Thirty-five people, including some security personnel, had been killed by the time martial law was lifted in June but the toll has risen to at least 81 since then, including Habib, according to Wefaq.

Shias account for about 70% 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.

David Cameron resisted pressure on Saturday to call for the cancellation of the grand prix, insisting that it was a matter for the F1 authorities whether the race should go ahead.

It came after Labour leader Ed Miliband and shadow home secretary Yvette Cooper added their voices to demands for the grand prix to be called off. Miliband said proceeding would send out the wrong signal at a time of protests over human rights abuses in the Gulf kingdom, while Cooper said British drivers should not take part.

The prime minister said there was "a process of reform under way in Bahrain" and added: "This government backs that reform and wants to help promote that reform."