The 62-year-old heir to the British throne and his 63-year-old wife, the Duchess of Cornwall, were said by palace officials to have been unharmed in the episode. The confrontation occurred when a group of about 50 protesters, some in full-face balaclavas, broke through a cordon of motorcycle police flanking the car as it approached London's theater district in slow-speed traffic. Some of the demonstrators shouted "off with their heads!" and others "Tory scum!"
A photograph of the couple, in formal evening dress, showed them registering shock as protesters beat on the side of their armored, chauffeur-driven Rolls-Royce with sticks and bottles, smashing a side window, denting a rear panel and splashing the car with white paint. A Jaguar tailing the car and carrying a palace security detail was so battered that the police ended up using its doors as shields.
Prime Minister David Cameron called the attack on the royal couple's car "shocking and regrettable."
Other violence across the city center continued into the night, with demonstrators trying to smash their way into the Treasury building at the heart of the Whitehall government district with makeshift rams made from steel crowd barriers, shouting "We want our money back!" The protesters set small fires and clashed with riot police officers and mounted units that formed cordons outside government buildings. BBC reporters at the scene wore helmets as the rioters threw shattered blocks of steel-reinforced concrete.
Scotland Yard said at mid-evening that at least 12 police officers were injured, six of them seriously, including one who was taken unconscious to the hospital after falling or being pulled off his horse. At that point, one large fire was still burning in front of the Palace of Westminster, seat of the House of Commons. At the height of the unrest, rioters threw snooker balls, lighted flares and fireworks at the police, and tried to topple statues in Westminster Square, across from the Commons. At least 43 were arrested.
The violence provided a disturbing backdrop to the day's political events, which were themselves a watershed moment for the seven-month-old coalition government of Mr. Cameron. Ahead of the parliamentary vote on the college fee increase, the government confronted a difficult rupture as the Liberal Democrats, who are the coalition's junior partners, split among themselves, raising questions over the coalition's long-term survival.
Although half of the Liberal bloc in the House of Commons voted against the tuition fee increase, the coalition won the vote by a margin of 323 to 302 votes. The 21-vote margin was far short of the coalition's nominal 84-vote majority, and threatened at one point to dwindle even further as Liberal leaders considered abstaining in a bid to keep their party together. In the end, the Liberal leader Nick Clegg and other Liberal ministers voted for the increase, though two Liberal ministerial aides resigned.
Without the continued backing of the Liberals, Mr. Cameron's Conservatives would likely have to face a new election, with no certainty they could win a contest that would be sure to center on the austerity program.
The Labour Party, loser in the May election, has a new leader in Ed Miliband, who replaced the former prime minister, Gordon Brown. It has focused its stand on opposition to the scale of the spending cuts, saying they hit hardest at the poor and risk pitching Britain back into recession.
A witness at the scene described the royal vehicle turning up Regent Street, one of London's main shopping thoroughfares, and heading into a crowd of protesters massed across the street, some of them smashing shop windows and setting fires. "I thought it was mad of them to head up Regent Street, because there were thousands of protesters at the top of the street, and the car was heading straight into them," the witness told the BBC.
He said Prince Charles and the duchess remained in their vehicle throughout and ultimately relaxed after a moment when the duchess, looking terrified, slid into the footwell beside the door. "He remained absolutely calm, he was beaming, as Camilla was," the witness said. "People were just trying to have a chat with them."
When the car moved on to the London Palladium, where Prince Charles and his wife were the guests of honor at an annual pre-Christmas variety show, photographs showed the royal couple smiling broadly. "There's a first time for everything," the duchess told reporters before the couple drove off in an armored police truck after the performance.
Student protests have been a vehicle for wider popular resistance to the 20 percent across-the-board cuts in government spending announced by the Cameron government in October, with tens of thousands of young people angered by a doubling, or potentially even a tripling, of government-regulated tuition fees, to a maximum of about $14,200 a year.
Under the new fees, which are to take effect in 2012, many students are expected to accumulate loans of as much as £40,000, about $63,000, during a three-year degree course. Part of what has stoked anger over the increases is that Britain's universities traditionally charged no tuition fees, with tuition costs met out of taxpayer grants to colleges or endowment funds.
The Labour government stirred its own wave of protest under Prime Minister Tony Blair, imposing a tuition fee ceiling of about $5,200. The Cameron government has cut university funding by about 80 percent, shifting the burden to the students.
The street clashes on Thursday raised concerns that the protests could be the template for even wider disturbances in the future, as the spending cuts and expected job losses begin to bite. The Liberals, with 57 parliamentary seats to the Conservatives' 308, had promised during the May election not to support a tuition fee increase, but made a U-turn once in government, saying the size of the deficit inherited from Labour made the increases necessary. With the Conservatives, they drew up a new schedule for the repayment of government-backed student loans, saying students will not have to begin paying back the loans until they earn at least $33,100.
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