Police unleashed tear gas and chemical-laced water Saturday at thousands of demonstrators who staged one of Malaysia's largest street rallies in years, demanding fair rules for national elections expected soon. Authorities were seen detaining dozens of people. Officials said three demonstrators and 20 police were injured. At least 25,000 demonstrators swamped Malaysia's largest city, hoping to pressure Prime Minister Najib Razak's ruling coalition to overhaul electoral policies before polls that could be held as early as June. Authorities insist the elections will be free and fair, rejecting activists' claims that the Election Commission is biased and that voter registration lists are tainted with fraudulent names. The demonstration remained peaceful for several hours, prompting organizers to declare it a success and ask people to head home. But when a small group appeared to suddenly breach the police barriers, authorities began firing tear gas and water laced with stinging chemicals at the crowd. Baton-wielding police backed by trucks mounted with water cannon sporadically fired tear gas at some demonstrators for at least an hour before much of the crowd was dispersed. People fled into streets and stores nearby, leaving shoes, bottles and other belongings scattered on the ground. Authorities were seen detaining dozens of people, with Malaysian media reports saying as many as 60 were arrested. Police said one protester snatched a pistol from its personnel during the chaos, though the weapon was later recovered, and others destroyed public property. Video footage by independent news website Malaysiakini showed angry demonstrators overturning a police car that allegedly hit two people. Home Minister Hishammuddin Hussein said 20 policemen and three demonstrators received treatment for unspecified injuries. He insisted that police acted “with utmost restraint,” but opposition leaders and rights groups said the use of tear gas was unjustified. “By launching a crackdown on peaceful protesters on the streets of Kuala Lumpur, the Malaysian government is once again showing its contempt for its people's basic rights and freedoms,” said Phil Robertson, Human Rights Watch's deputy director for Asia.