KIEV, Ukraine (AP) -- Thousands of anti-government demonstrators clashed for hours with riot police in Ukraine's capital Sunday, attacking officers with sticks, stones and flares after new laws were passed to stifle protests. Dozens of people, including more than 20 police, were injured.
The protesters, many wearing hard hats and gas masks in defiance of the new legislation, also used stun grenades and fire extinguishers on officers. A police bus was set on fire, and some activists broke pavement into chunks.
Police responded by using tear gas and stun grenades of their own. Water cannons were also fired at the protesters in temperatures of -8 C (18 F), but the clashes continued.
The violence seriously escalated Ukraine's political crisis, which erupted two months ago after President's Viktor Yanukovych's decision to freeze ties with the European Union and seek a huge bailout from Russia.
The decision set off round-the-clock protests in Kiev's center, where demonstrators built a large tent camp. The protests have been mostly peaceful, but anger rose sharply after Yanukovych on Friday approved laws sharply limiting Ukrainians' rights to protest, civic activism and free speech. The U.S. called that legislation "undemocratic."
The laws prohibit demonstrators from wearing masks or hard hats at rallies. Several opposition leaders addressed the crowds from a giant stage, wearing bright construction workers' hats. Other provisions of the controversial legislation restrict the activity of non-governmental groups funded by the West and seek to equate critical reporting with defamation.
The harsh new laws brought a crowd of tens of thousands to the protest at Kiev's central square on Sunday.
While most remained on the square, a group of radicals marched toward a police cordon several hundred meters away blocking an area housing government offices and began attacking riot police with sticks to push their way toward Ukraine's parliament building. The crowd then swelled to thousands.
The blasts of stun grenades echoed and plumes of smoke rose above the crowd. Activists chanted "Shame!" and "Revolution." The Interior Ministry said more than 20 police were injured, four of them seriously; there were no immediate figures for protester injuries.
The ministry also said a criminal case had been opened on charges of mass disorder; convictions under that charge could bring prison sentences of up to 15 years.
Opposition leader Vitali Klitschko spent hours attempting to stop the protesters from attacking police, but he himself was sprayed with a fire extinguisher.
Klitschko's top allies, who stood by his side at the peaceful rally earlier in the day, didn't show up at the site of the clashes for the most of the day. Instead, they called for a peaceful means of protest from nearby Independence Square and condemned the clashes.
"No power in the country is worth losing at least one human life," protest leader Arseniy Yatsenyuk said from the stage as the clashes dragged late into the evening. "That is why I condemn the violence that took place just now."
Scores of opposition leaders and journalists have been attacked, harassed and prosecuted, since the anti-government protests started Nov. 21.
Yanukovych's government has ignored previous demands made by the opposition.
Opposition leaders denounced Yanukovych's legislation as unconstitutional and called for the formation of parallel governing structures in the country.
"The power in Ukraine belongs to the people," Yatsenyuk said.