David Cameron resisted pressure to call for the cancellation of the Bahrain Grand Prix today, insisting it was a matter for the Formula One authorities whether the race should go ahead on Sunday.
Labour leader Ed Miliband added his voice today to demands for the race to be called off, and urged the Prime Minister to do the same.
Mr Miliband said it would send out the wrong signal for the Grand Prix to go ahead at a time of protests over human rights abuses in the Gulf kingdom.
But the Prime Minister said it was "a matter for Formula One", adding: "It's important that peaceful protests are allowed to go on."
Speaking during a visit to Preston, Mr Cameron 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."
Some 17 MPs have signed a cross-party motion at Westminster calling for the race to be called off, warning that it will be used by the Bahrain government as "an endorsement of its policies of suppression of dissent".
Last year's Grand Prix was cancelled amid international condemnation of the bloody crackdown by state security forces on pro-democracy protests, which killed at least 50 people.
Additional security troops were deployed around the Bahrain International Circuit (BIC) and across the capital Manama today as the three-day event leading up to Sunday's race got under way.
Two members of the Force India team have flown home after a car was caught up in a petrol bomb incident on Wednesday, and the team's drivers Niko Hulkenberg and Paul Di Resta are to cut practice sessions short today to be able to leave the desert circuit before nightfall.
But Formula One supremo Bernie Ecclestone said: "I can't call this race off. Nothing to do with us. We've an agreement to be here, and we're here."
Opponents of the kingdom's Sunni monarchy - many of them drawn from the country's Shi'ite majority - are expected to rally in an area close to the BIC later today.
Speaking during a visit to south Wales, Mr Miliband said: "I certainly think it is the case that, given the violence we have seen in Bahrain and given the human rights abuses, I don't believe the Grand Prix should go ahead.
"I hope that the Government will make its view clear and say the same.
"Sport and politics generally shouldn't mix, but... what kind of signal does it send to the world when this Grand Prix is going ahead, given the concerns there are, given the violence we have seen in Bahrain, given the continuing issues around human rights?"
Downing Street said the Government remained "concerned" about the violence in Bahrain, but refused to be drawn on whether the race should go ahead.
"It is not for us to dictate what sporting events happen in other countries," a No 10 spokeswoman said.
Shadow home secretary Yvette Cooper urged British Formula One stars Jenson Button and Lewis Hamilton to pull out of this weekend's controversial event.
"It shouldn't go ahead, I don't think British drivers should go, I think the Formula 1 should not go ahead in Bahrain," Ms Cooper told BBC1's Question Time.
But McLaren driver Button refused to become embroiled in the issue during an interview yesterday, telling reporters: "I'm not going to get into the details of it. You are here interviewing me as a driver and that's exactly what I am going to talk about - motor racing. The outside issues, I'm not going to talk about."
Respect MP George Galloway said the tracks of the Middle Eastern race circuit were "stained by the blood of the people who are asking for a vote", adding that "anyone who drives over them will never be forgiven."
And Conservative former shadow home secretary David Davis said: "The only ethical option for Formula 1 is not to continue with the Bahrain Grand Prix.
"Regrettably, this is an example of where big money is over-ruling serious ethical concerns."
The Foreign Office (FCO) has advised British motor racing fans against travelling to the Grand Prix.
The Bahrain International Circuit is only 25 miles away from the capital Manama, which has been the scene of violent clashes over recent weeks.
In a recent report, Amnesty International said security forces were still using excessive and unnecessary force against anti-government protesters.
Amnesty said it was receiving reports of the torture and ill-treatment of demonstrators, who have been involved in clashes with police.
Bahrain's Crown Prince Salman al-Khalifa said that cancelling the race would "empower extremists".
The parliamentary motion calling for it to be cancelled "certainly doesn't represent the entire British political system", he said.
He compared the protests taking place in Bahrain to the riots in England last summer: "It goes to show there are people out to cause chaos. You had these problems last year in your country.
"There is a big difference between protesting for political right and rioting, and the attack that happened around Force India was aimed at the police, was unprovoked and quite dangerous. At no time was anyone from Formula One in danger."
Prince Salman said: "I think this race should continue because it is indeed a very big event for this country, important economically, socially.
"Political parties from the whole spectrum, both conservative and opposition, have welcomed the race."
He added: "I also think cancelling the race just empowers extremists.
"For those of us trying to navigate a way out of this political problem, having the race allows us to build bridges across communities, to get people working together.
"It allows us to celebrate our nation. It is an idea that is positive, not one that is divisive."
Standing alongside the Crown Prince, Mr Ecclestone told reporters: "What happens in this country is nothing to do with us.
"As his royal highness said, we have people in all sorts of countries not satisfied, people in England - Conservative and Labour - that don't agree with things.
"That's how it is. That's how the world is.
"But this race has given the protesters an incredible platform for all you guys to talk to them.
"They say they talk about democracy, which is freedom of speech. They've had all the freedom in the world to talk to you guys."
Chairman of the Commons Foreign Affairs Committee Richard Ottaway told Sky News: "I'm not one of those calling for the Grand Prix to be stopped and I don't think the British Government should call for it to be stopped.
"There are very serious issues here, but I think it is very dangerous to mix politics with sport. China, who had the Grand Prix last weekend, has a very bad human rights record, yet there were no calls for that to be called off."