Malaysian police today fired tear gas and water cannons at a huge opposition rally in Kuala Lumpur, BBC News reported.
Thousands of protesters, many wearing the signature yellow of the reform movement, gathered in Merdeka (Independence) Square in defiance of a ban on political demonstrations there, according to Agence-France Press.
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The protest, which one police official told BBC was some 25,000 strong, is one of the largest seen in the former British colony in decades.
Some 20 people were reportedly arrested as the government moved to disperse the crowds, according to BBC.
Human Rights Watch's deputy Asia director Phil Robertson condemned the violence as proof that "the Malaysian government is once again showing its contempt for its people's basic rights and freedoms," reported Reuters.
Demonstrators are demanding reforms of the country's electoral system ahead of coming elections, said BBC.
Reuters said the vote could be held as early as June, but today's events could delay the date.
The government has come under criticism for its brutal crackdown on a pro-reform rally last July, said Reuters.
Malaysia is predominantly Muslim but has a diverse population with a history of racial tension pitting the country's Chinese community -- about a quarter of the population -- against the ethnic Malays.