Opposition supporters in Bahrain are attending a mass protest demanding an end to the crackdown on dissent and the cancellation of Sunday's Grand Prix.
Thousands of people were said to be walking from Budaiya, a town west of Manama, down the motorway towards the Bahrain International Circuit (BIC).
One group has threatened "three days of rage" to coincide with the race.
Earlier, Crown Prince Salman bin Hamad Al Khalifa warned that cancelling the Grand Prix "just empowers extremists".
While admitting that Bahrain was "not perfect", he told journalists after Friday's first practice session that the race was a "force for good".
"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, get people working together," he said.
Formula 1 boss Bernie Ecclestone added: "If people have got a complaint about something else, it's nothing to do with F1."
Prince Salman also guaranteed the safety of the racing drivers and teams, after a car carrying members of the Force India team was caught in clashes between police and anti-government protesters on Thursday, and a petrol bomb exploded nearby.
Force India's deputy team principal earlier said its involvement in the day's second practice session would be limited due to safety concerns.
Formula 1's governing body, the FIA, only went ahead with the race after the government said it had security under control.
At least 5,000 people were said to have gathered in Budaiya on Friday afternoon for a protest organised by the main Shia opposition group, al-Wefaq, although some put the turnout in the tens of thousands.
It was sanctioned by the authorities despite overnight clashes between security forces and protesters in several mainly Shia villages.
Riot police fired tear gas and stun grenades at the protesters, who responded by throwing petrol bombs and stones, witnesses said.
"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," public security chief Maj-Gen Tariq al-Hassan said in a statement.
But one member of the Bahrain Youth Society of Human Rights, Mohammed, told the BBC they had thrown petrol bombs in self-defence.
The overnight demonstrations called for the "overthrow of the regime" and the release of the human rights and political activist, Abdulhadi al-Khawaja, who has been on hunger strike in prison for more than 70 days in protest at the life sentence he received from a military tribunal in June.
Mr Khawaja stopped drinking water on Thursday and called a lawyer to write his will, according to his daughter, Zaineb.
"My father said: "If I die, in the next 24 hrs, I ask the [people] to continue on path of peaceful resistance," she wrote on Twitter on Friday afternoon. "[Second]: he asks that nobody attempts to go on a similar strike til death."
"My father continued: '... I don't want anybody to be hurt in my name,'" Ms Khawaja added.
Bahrain's highest court, the Court of Cassation, is due to rule on Mr Khawaja's appeal against his conviction on Monday - a day after the Grand Prix.
Al-Wefaq reported that 70 people had been injured by security forces in the past two days and 80 others arrested.
"The protests are having international impact because there are foreign journalists here for F1. Once they leave, the repression will continue but the world won't hear about it," activist Nabeel Rajab told the BBC.
The government has called on Bahrainis to move forward and hopes the race will show that life is returning to normal.
It says King Hamad has accepted the findings of an independent panel of human rights experts that investigated last year's unrest, and announced constitutional reforms intended to lead to greater accountability.