Anti-government protesters flooded a main highway in a march stretching for miles and security forces fired tear gas in breakaway clashes Friday as Bahrain's leaders struggled to contain opposition anger while under the world's spotlight as the island nation hosts the Formula One Grand Prix.

The government allowed the massive demonstration in an apparent bid to avoid the hit-and-run street battles that are the hallmark of the Gulf nation's 14-month uprising _ and an embarrassing spectacle for Bahrain's Western-backed rulers as F1 teams prepare for Sunday's race.

But violence flared as small groups in the march peeled away from the route to challenge riot police, who answered with volleys of tear gas and stun grenades. Some protesters sought refuge in a mall and nearby shops about 20 kilometers (12 miles) north of the Formula One track, where practice runs took place and Bahrain's crown prince vowed the country's premier international event would go ahead.

Last year, a wave of anti-government protests by the island's Shiite majority and a crackdown by the Sunni rulers forced organizers to cancel the 2011 Bahrain GP. At least 50 people have been killed since the start of Bahrain's uprising _ the longest-running in the Arab Spring _ which seeks a greater political voice for Shiites and to weaken the near monopoly of the Sunni dynasty that has ruled for more than 200 years.

"We demand democracy" and "Down, Down Hamad," chanted some of the tens of thousands of opposition supporters in reference to King Hamad bin Isa Al Khalifa, as they massed on the main highway leading out of the capital, Manama. Bahrain's monarchy is the main backer of the F1 race, and the crown prince owns rights to the event.

Hours before the march, Bahrain's most senior Shiite cleric, Sheik Isa Qassim, delivered a strongly worded sermon that denounced authorities for making dozens of arrests of suspected dissidents in recent weeks. He called the intensified crackdowns before the F1 event "as if we are entering a war."

Bahrain's rulers lobbied hard to stage this year's Grand Prix as part of attempts to portray stability in the strategic kingdom, which is home to the U.S. Navy's 5th Fleet. On the opposite side, rights groups and others campaigned to keep the race away, citing the relentless pressures by security forces and the imprisonment of opposition figures _ including a Shiite political activist on a more than a two-month-long hunger strike.

The U.S.-based group Physicians for Human Rights, also said it was concerned about the near daily use of tear gas in Bahrain, including in crowded urban areas and homes, and its possible long-term health consequences, including increased rates of miscarriages and birth defects.

"Despite promises of reform since our investigation into the Kingdom last year, the Government's excessive use of force has only increased," said the group's deputy director, Richard Sollom.

The hacking collective Anonymous, meanwhile, claimed responsibility for a denial-of-service attack on the official Formula One website in protest over the running of the Bahrain Grand Prix this weekend. Such web attacks work by overwhelming a site with bogus traffic.

Shiites account for about 70 percent of Bahrain's population of just over half a million people, but claim they face widespread discrimination and lack opportunities granted to the Sunni minority. The country's leaders have offered some reforms, but the opposition says they fall short of Shiite demands for a greater voice in the country's affairs and an elected government.

The unrest has put Washington into an awkward position. U.S. officials have called for efforts to reopen political dialogue in Bahrain, but are careful not to press too hard against the nation's leadership and possibly jeopardize its important military ties.

In Washington, State Department spokeswoman Victoria Nuland expressed the Obama administration's concern about "the increase in violence in Bahrain, especially leading up to the Formula 1 race."

"These are unproductive, unhelpful acts in building the kind of meaningful trust and reconciliation that is needed in Bahrain," Nuland told reporters. "We're calling for, again, Bahraini government respect for universal human rights and demonstrators' restraint in ensuring that they are peaceful."

Bahrian's crown prince, Salman bin Hamad Al Khalifa, visited the track Friday and rejected any suggestion that the race should be scrapped.

"I think canceling the race just empowers extremists," he said. "For those of us trying to navigate a way out of this political problem, having the race allows us ... to celebrate our nation as an idea that is positive, not one that is divisive."

Clashes take place nearly every day with demonstrators hurling firebombs and riot police responding with tear gas and sometimes firing birdshot. The main Shiite political group, Al Wefaq, says at least 50 people have been injured in the past two days when security forces fired pellets to disperse protesters.

Additional security forces deployed this week, setting up checkpoints Friday on roads leading to the Bahrain International Circuit and increasing their presence across Manama.

The rulers have depicted the race _ expected to draw a worldwide TV audience of about 100 million in 187 countries _ as an event that will put the divided society on the path of reconciliation.

"I genuinely believe this race is a force for good, it unites many people from many different religious backgrounds, sects and ethnicities," said the crown prince.

In the past weeks, however, much of the protesters' anger has been directed at the crown prince, who is also the commander of the kingdom's armed forces, which the opposition supporters say have been enforcing the crackdown.

Last year, Salman was tasked to lead a national dialogue aimed at reconciliation between Shiite and Sunnis. The talks broke down without any compromise and have not yet resumed.

In Iraq, hard-line Shiite cleric Muqtada al-Sadr, denounced Bahrain for staging the grand prix while "blood is being shed" on the island. Al-Sadr also condemned the F1 teams for racing, saying their presence in Bahrain gives "support for injustices and the killings."

As a majority Shiite country, Iraq has backed Bahrain's Shiite-led protests.

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Associated Press writers Brian Murphy in Dubai, United Arab Emirates, and Bushra Juhi in Baghdad contributed to this report.