TEHRAN (Reuters/AFP) - Iranian riot police on motorcycles beat supporters of presidential challenger Mir Hossein Mousavi who were protesting on Saturday against President Mahmoud Ahmadinejads disputed election win. Police also fired teargas at rioters in Tehran as protesters swept through the Iranian capital, some pelting stones at baton-wielding policemen. Angry crowds first emerged near Mousavis campaign office in central Tehran, where protesters, including women, were hit with sticks as riot police on motorbikes moved in to break up the gathering, an AFP correspondent said. A Reuters reporter said she and others were beaten by police with batons as police chased and arrested demonstrators staging a sit-in at Tehrans Vanak Square, one of the capitals busiest intersections. At least three people were injured in the clash, which broke out after the Interior Ministry announced the hardline incumbents resounding victory in Fridays vote. Tehrans deputy police chief, Mohsen Khancharli, said the force would strongly confront any gathering or rally held without permission. He said people protesting against the election outcome had damaged cars. Police are not confronting people but only those who are disturbing public order or who make damage to public places, he told the official IRNA news agency. Up to 2,000 Mousavi supporters took part in the sit-in in the middle of the road, chanting: Mousavi take back our vote What happened to our vote? Hundreds of the former prime ministers backers later gathered in side streets near Vanak, chanting anti-Ahmadinejad slogans and bringing traffic to a standstill. We are Iranians too, and Mousavi is our president, they shouted. The protests were a first reaction by Mousavis supporters, who thronged the streets of Tehran nightly in the run-up to Fridays vote, to Ahmadinejads victory. Some demonstrators set rubbish bins on fire and taunted riot police. Others threw stones at them. Police charged at the protesters who then fled, only to return shortly afterwards. Down with the dictator Mousavi, retrieve our votes for us shouted the crowds as they streamed into Tehrans main squares, with violence erupting in some areas on a scale not seen since student riots a decade ago. The election was corrupt. If Ahmadinejad had won where are his supporters? How come they are not celebrating? said Reza, 25. Most people think Mousavi won the election, said another young man, Saman. They gave only their first names. Clashes between police and Mousavi supporters also took place in other areas of the capital, witnesses said. Reuters photographs showed a burning bus in downtown Tehran, as well as motorbikes on fire. Elsewhere in Tehran, tyres were set ablaze, witnesses said. A group of angry supporters even beat a policeman badly, leaving him with blood streaming from his face, the correspondent said. The policeman tried to seek refuge in a nearby house but the residents pushed him out. Some people who tried to escape the clouds of teargas were chased by police into nearby buildings where they were beaten despite pleas to be set free; others were given shelter by local residents. Scuffles also broke out early on Saturday between police and chanting Mousavi supporters in another Tehran square. Police say they have increased security across the capital to prevent trouble.