Police fired tear gas and detained hundreds of activists as more than 10,000 demonstrators massed across Malaysia's largest city demanding electoral reforms in the country's biggest political rally in years.
The opposition-backed rally in Kuala Lumpur was the culmination of weeks of intense pressure on Prime Minister Najib Razak's long-ruling coalition to make election laws fairer and more transparent ahead of national polls widely expected by mid-2012.
Demonstrators marched in defiance of Najib's administration, which has declared the rally illegal and warned people repeatedly to avoid it.
Opposition leaders accuse Mr Najib's National Front of relying on fraud to preserve its 54-year grip on power, which has been eroded in recent years amid mounting complaints about corruption and racial discrimination. The government insists the current electoral policies are even-handed.
Authorities took extraordinary security measures to deter Saturday's rally by sealing off roads, closing train stations and deploying trucks mounted with water cannons near the Independence Stadium in central Kuala Lumpur, where activists sought to gather.
The federal police force said in a statement that it had detained 514 people in a clampdown called Operation Erase Bersih, referring to the Bersih coalition of civic groups organising the rally. Many were held at public areas where they had gathered to begin their walk to the stadium.
Thousands of others were trying to reach the stadium from various parts of Kuala Lumpur, chanting "Long live the people" and carrying yellow balloons and flowers as they marched. Most activists said the total number of demonstrators exceeded 10,000 people, making it Malaysia's biggest street rally since 2007.
The Bersih coalition said tear gas was fired at at least one crowd of several hundred, but no injuries were immediately reported. Tense stand-offs were reported at other locations where riot police armed with batons tried to prevent people from advancing.
The rally has galvanised the opposition and has been credited for a surge in political awareness among the public in recent weeks.
Government officials accuse opposition leader Anwar Ibrahim's three-party alliance of endorsing the rally to cause chaos on the streets and undermine the National Front.