CAIRO (Reuters) - Police fought with thousands of Egyptians who defied a government ban on Wednesday to protest against President Hosni Mubarak's 30-year-old rule, firing tear gas at the crowds and dragging away demonstrators. Protesters burnt tyres and hurled stones at police as groups gathered at different parts of the capital Cairo. The scenes were unprecedented in the country, one of the US' closest Middle East allies, and follow the overthrow two weeks ago of another long-serving Arab strongman, Tunisian leader Zine al-Abidine Ben Ali, in a popular revolt. Activists had called on people to rally again after a "Day of Wrath" on Tuesday of anti-government demonstrations across Egypt in which three protesters and one policeman were killed. Security forces have arrested about 500 demonstrators over the two days, an Interior Ministry source said. Witnesses said officers, some in civilian clothes, hauled away people and bundled them into unmarked vans on Wednesday. Police fired shots into the air near the central Cairo court complex, witnesses said. In another area, they drove riot trucks into a crowd of about 3,000 people to force them to disperse. The police also fired rubber bullets at protesters in central Cairo and officers in plain clothes dragged protesters away from the crowd and beat them with batons, witnesses said. A protester in the centre of Cairo told Reuters: "The main tactic now is we turn up suddenly and quickly without a warning or an announcement. That way we gain ground." Social networking sites such as Twitter and Facebook have been a key means of communications for the protesters. Egyptians complained Facebook and Twitter were subsequently blocked but the government denied it. A Facebook spokesman in London said it had not seen any major changes in traffic from Egypt. Twitter confirmed its site was blocked on Tuesday, although users could still access it via proxy sites. The coordinated protests were unlike anything witnessed in Egypt since Mubarak came to power in 1981 after President Anwar Sadat was assassinated by Islamists. The demonstrators complain of poverty, unemployment, corruption and repression and, inspired by the Tunisian revolt, demand that Mubarak step down. The US said Egypt was still a "close and important ally". But US Secretary of State Hillary Clinton also urged the government to allow peaceful protests and not to block the social networking sites. "We believe strongly that the Egyptian government has an important opportunity at this moment in time to implement political, economic and social reforms to respond to the legitimate needs and interests of the Egyptian people," she said.