Bahraini police clashed on Sunday with Shiite demonstrators, only hours ahead of the Gulf state's Formula One Grand Prix race, which the authorities insisted would go ahead without disruption.
Nico Rosberg of Mercedes will start on pole after outpacing fellow German and championship leader Sebastian Vettel of Red Bull in qualifying on Saturday. Spain's Fernando Alonso of Ferrari is third on the grid.
But the racing has been overshadowed by sometimes bloody clashes away from the circuit which have dragged Formula One bosses into controversy for a second year in a row.
Police have fired birdshot and tear gas to contain simmering resentment at a deadly crackdown by the Sunni royal family on Arab Spring-inspired protests that erupted two years ago led by the kingdom's Shiite Muslim majority.
The scenes have renewed pressure on motor racing bosses over their decision to keep Bahrain on the Formula One calendar despite similar disturbances that clouded the event last year.
At dawn on Sunday, masked youths set alight tyres on roads in Shiite villages just outside Manama, but access was still open to the Sakhir circuit, south of the capital, witnesses reported.
Overnight, police fired tear gas to disperse demonstrators in Shiite villages who chanted against the "race of blood," said the witnesses.
Armoured security vehicles, police cars and armed guards at road blocks decorated the route into Sakhir in the morning.
Queues at the circuit confirmed a heavy security presence as every vehicle slowed to pass through a single file inspection by gun-carrying security guards.
"Police are out in force to beef up security measures at the Bahrain International Circuit," said Major General Tariq Hassan, the country's public security chief.
"Security forces are deployed in all regions countrywide, undertaking pre-emptive and precautionary measures to ensure citizens and residents’ safety and security and protect public and private properties."
Hundreds of Shiite demonstrators who attempted on Saturday evening to reach Manama's Pearl square, the epicentre of 2011 protests, were forcibly dispersed.
Police fired tear gas and some protesters retaliated with petrol bombs.
Hundreds had also taken to the streets of Shiite villages overnight Friday, prompting clashes.
"Your race is a crime," chanted protesters armed with petrol bombs and stones. "No, no to the race of blood."
Police responded with tear gas and stun grenades.
Talking to F1 reporters at Sakhir, Crown Prince Salman bin Hamad al-Khalifa admitted there were "issues" affecting the kingdom, which was rocked by deadly unrest in the 1990s and has been hit by renewed disturbances since 2011.
He dismissed criticism that Bahrain was exploiting the Grand event in a bid to boost its image and economy.
"We've never used this race to say that everything's fine," said Prince Salman. "We recognise there are issues in the country but they are to be solved in a political process which is well underway."
The prince, regarded as a moderate, said he was "optimistic" a settlement could be reached with the Shiite-led opposition and acknowledged protesters against the race had largely refrained from violence.
"I think they were largely peaceful -- people expressing their rights to disagree," he said.
Human rights groups say the political violence in Bahrain has killed at least 80 people since 2011.
Formula One supremo Bernie Ecclestone said, however, that it was not for the sport to take a moral stand.
"We're not here, or we don't go anywhere, to judge how a country is run," Ecclestone told the BBC.
"Human rights are that the people that live in a country abide by the laws of that country," he said. "So it's a case of whatever the laws are in a country. People need to respect them."
Strategically located just across the Gulf from Iran, Bahrain is home base for the US Fifth Fleet and also a major offshore financial and services centre for its oil-rich Gulf Arab neighbours.