Bahrain's Crown Prince rejected calls from human rights activists to cancel this weekend's Grand Prix as pro-democracy opposition groups threatened further protests after days of clashes with security forces.

As Formula One cars took to the Sakhir circuit for practice, Crown Prince Salman said that calling off the race on Sunday would play into the hands of extremists.

Speaking to the media alongside Formula One chief Bernie Ecclestone and in front of TV crews broadcasting live around the world, Salman said: I think cancelling the race just empowers extremists.

The government aims to use the Grand Prix as a way of showing that life is back to normal after democracy groups launched an Arab Spring-inspired uprising last year. The protests were initially crushed, but were not stamped out, and demonstrations and clashes are frequent.

Human rights organisations and anti-government groups have argued that the race should not be held while what they describe as political repression and rights abuses are taking place in Bahrain.

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, the crown prince said.

The prince was speaking after two of the 12 teams said their staff had been caught up in incident where protesters threw petrol bombs on their way back from the track to hotels in Manama.

The Force India team missed Friday's second practice for safety reasons to make sure staff could get back to their hotels safely before nightfall.

On the eve of Friday's practice session, protests had flared in villages surrounding the capital, Manama, away from the race circuit. Police fired tear gas and stun grenades to disperse demonstrators in clashes that have been building in the week leading to Sunday's round of the World Championship.

Bahrain has been in turmoil since a democracy movement erupted last year following uprisings in Egypt and Tunisia. Protests were initially crushed with the loss of dozens of lives, but youths still clash daily with riot police in Shi'ite Muslim districts, and thousands take part in opposition rallies.

Bahrain's ruling al-Khalifa family, a Sunni Muslim dynasty ruling a majority Shi'ite population caught between neighbours Saudi Arabia and Iran with opposite sympathies in its internal strife, hopes the race will offer an opportunity to tell the world that life is returning to normal.

Unrest forced the cancellation of last year's Grand Prix, and the 2012 race has been in doubt as Bahrain's human rights record has come under fire from abroad.

The Force India team cancelled its second practice on Friday, choosing to stay in its hotel because of security fears.

On Friday the British-based team saw burning petrol bombs on the road back to their hotel after an apparent clash between police and protesters. Two team members asked to go home.

A number of rioters and vandals had been arrested for taking part in illegal rallies and gatherings, blocking roads and endangering people's lives by attacking them with petrol bombs, iron rods and stones, a government statement said, citing police chief Tariq Al Hassan.

The Sauber team also said it encountered youths with petrol bombs in Thursday's drive home.

Bernie Ecclestone, the colourful commercial supremo of the Formula One franchise who arrived at the track on Friday, described general security fears as nonsense and said Force India had been offered a security detail.

Only small crowds were seen in the grandstand on Friday for an event that has cost Bahrain an estimated $40 million to stage. The Grand Prix drew 100,000 visitors to the nation of just 1.3 million and generated half a billion dollars in spending when it was last held two years ago.

The opposition is taking the chance to press their case while the world is watching.

Opposition parties led by Shi'ite group Wefaq planned to stage a march in a mainly Shi'ite residential district outside Manama later on Friday to demand democratic reforms in a country where the ruling family dominates government.

Other protesters could attempt to reach the main highway between Manama and the race track.

The leading Shi'ite cleric Sheikh Isa Qassim attacked the government in a sermon on Friday for ignoring popular demands.

This is the crisis of a government that does not want to acknowledge the right of people to rule by themselves and choose their representatives, he said.

POLICE LOCKDOWN

Manama is under tight security, with dozens of armoured vehicles stationed around the capital and the road to the Bahrain International Circuit in Sakhir. Activists say barbed wire has installed near some parts of the main highway.

Though life in Manama's main commercial, residential and tourist districts appears detached from the nightly battles, tear gas often floats in from conflict zones around the capital.

The death toll from the year of turmoil has risen to around 70, activists say, with many due to heavy use of tear gas. The government disputes the causes of death and accuses protesters in Shi'ite villages of being saboteurs out to harm the police.

Activists say riot police are trying to lock Shi'ites down in their villages to stop them gathering on main highways.

They say around 95 protest organisers have been arrested in night raids in the past week and 54 people wounded in clashes, with heavy use of birdshot. Police have declined to give figures on arrests and injuries.

They are trying to frighten people after they realised that tear gas did not make the protests to stop, said senior Wefaq member Matar Matar. I think if they fail to stop the protests by this method too, they may reach to the level of carelessness towards killing.

While sports journalists poured in to cover the race, non-sports reporters from Reuters and some other news organisations have not been granted visas to visit the Gulf island.

Bahrain wants the international attention brought by hosting a Grand Prix but doesn't want foreign journalists to wander from the race track where they might see political protests, said Robert Mahoney, deputy director of the Committee to Protect Journalists in New York.

Ecclestone said he was not aware of the visa rejections faced by some journalists but said he would investigate.

Bahrain is the base for the U.S. Navy's Fifth Fleet, among whose tasks is deterring Iran from making good on recent threats to disrupt Gulf oil tanker routes to the West.

Washington has only gently prodded Bahrain's rulers to improve their human rights record and push forward political reforms, and does not want to jeopardise ties with a ruling family it views as an ally in the region.

The F1 media spotlight will only highlight the ongoing troubles Bahrain faces in the absence of any serious attempts at political compromise, wrote Chatham House analyst Jane Kinninmont in online magazine Foreign Policy.

(Writing by Andrew Hammond; Editing by Will Waterman and Giles Elgood)