Thousands of people protested in a Shiite suburb of Bahrain's capital on Formula One practice day as the Gulf kingdom's crown prince insisted Sunday's race would go ahead to avoid "empowering extremists."

Demonstrators gathered late Friday afternoon in the Budaya area four kilometres (2.5 miles) west of Manama chanting "Down with the dictatorship!" and "Revolt until victory!" witnesses said.

Some wore T-shirts bearing the slogan "Ready to die for Bahrain".

Al-Wefaq, the main opposition group in the Shiite-majority but Sunni-ruled kingdom, said anti-riot police intervened "violently" and several demonstrators were injured.

Earlier, the crown prince ruled out cancelling the event.

"I think cancelling just empowers extremists," Salman bin Hamad Al-Khalifa said during a media briefing at the Sakhir circuit.

"I think for those of us who are trying to navigate a way out of this political problem, having the race allows us to build bridges across communities, and get people working together."

His remarks came after Force India said that, "for logistical reasons," it would not take part in Friday's second practice "to ensure the most competitive performance in FP3, qualifying and the race."

Earlier, deputy team principal Bob Fernley told Autosport magazine they were considering curtailing practice in order to return to their hotel before dark but that they would not miss qualification on Saturday or Sunday's race.

Four Force India team members were caught up in a confrontation between protesters and police on Wednesday when a petrol bomb exploded near their car.

Two team members chose to leave Bahrain on Thursday.

And the Sauber team confirmed on Friday that they drove through a similar incident the previous day, but that none of the 12 mechanics in an unmarked mini-bus was hurt.

As second practice continued, Formula One's chief executive and commercial ringmaster Bernie Ecclestone appeared unmoved about escalating safety concerns.

He said he did not understand why Force India was so worried about safety, adding that he had personally offered to drive with the team from the circuit if they wanted reassurance.

"They have asked and been told they can have security if they want it," he said. "None of the other teams seem to have a problem."

McLaren team principal Martin Whitmarsh said all 10,500 seats in the main stand had been sold "which shows that the race is welcomed by many people at all levels of society. But often in this type of situation the voice of the majority is not heard."

Witnesses reported overnight violence in Shiite villages far from the Sakhir circuit.

"Eighteen people were wounded" when security forces fired buckshot and tear gas to disperse protesters, said rights activist Mohammed Maskati.

"The people want to topple the regime," dozens of protesters shouted as they carried pictures of jailed hunger striker Abdulhadi al-Khawaja. "Down Hamad," they cried, referring to Bahrain's king, witnesses said.

The protesters hurled petrol bombs at security forces, witnesses said.

The demonstrations were "a message to those taking part in the F1 race to bring their attention to human rights violations in Bahrain," Maskati told AFP, adding that "95 people have been arrested since April 14."

Amid unease among Grand Prix participants, with Force India's British driver Paul Di Resta acknowledging it was an "uncomfortable situation," Prince Salman accepted that some F1 teams had safety and security concerns.

"I absolutely can guarantee that any problems that may or may not happen are not directed at F1," he said. "It goes to show that there are people who are out to cause chaos.

"The attack that happened around Force India was aimed at the police. It was unprovoked and it was quite dangerous. But at no time was anyone from F1 in danger."

The February 14 Youth Movement has called on social networking sites for "three days of rage" to coincide with the race.

Al-Wefaq had called for a week of daily protests during the Grand Prix to focus media attention on their long-standing demands for reforms.

At a news conference on Friday, the head of the Ferrari team and Force India's number two expressed hopes the race would contribute to reconciliation in Bahrain.

Force India's Fernley said "hopefully, the Formula One programme has brought the world's media here, it gives a good platform for debate and hopefully it will help with the healing process for Bahrain, and that's why we're here."

Ferrari's Stefano Domenicali said: "There are a lot of things going on and we really hope that all the dialogue that has started within the different parts will do the best thing in the shortest time possible for everyone."

The Bahrain event was cancelled last year in the wake of and uprising and the government crackdown that followed in which a government commission said 35 people were killed.

Predominantly Shiite Iran, which has strained ties with Manama, called on Bahrain to "end the suppression" of its population.