At least 20,000 Malaysians defied government warnings by marching for electoral reforms Saturday, as police fired tear gas and detained more than 1,600 people in the country’s biggest political rally in four years.

The crackdown on the opposition-backed demonstration in Malaysia’s capital, Kuala Lumpur, triggered criticism that Prime Minister Najib Razak’s long-ruling coalition was unwilling to allow public dissent or make election laws fairer ahead of national polls widely expected by mid-2012.

Najib’s administration declared the rally illegal and repeatedly warned people to avoid it. More than 200 activists were arrested in the past two weeks for promoting the event.

Ahead of the rally, authorities blocked roads, shut rail stations and deployed water-cannon trucks near the stadium in downtown Kuala Lumpur where activists sought to gather.

Police said in a statement that they detained 1,667 people Saturday. Those arrested included several senior opposition officials. Some were released after several hours, with police indicating that most would not be held overnight.

Police fired numerous rounds of tear gas and chemical-laced water in repeated attempts to disperse the crowds. Witnesses said riot police armed with batons charged at some protesters and dragged them into trucks. Some were seen bleeding, but police could not confirm any injuries.

Anwar Ibrahim, Malaysia’s top opposition figure, was being treated at a hospital for a “minor injury” that he said he received when his group was hit by tear gas, local media said.

The demonstrators dispersed after a five-hour standoff with police. Only several hundred reached the stadium.

Organizers said 50,000 took part in the rally, but police said there were no more than 6,000. Other observers and participants put the total between 20,000 and 30,000.

Opposition leaders accuse Najib’s National Front coalition 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 that electoral policies are evenhanded.

The activists’ demands include an overhaul of voter registration lists, tougher measures to curb fraud, and fairer opportunities for opposition politicians to campaign in government-linked media. The National Front’s mandate expires in mid-2013, but many analysts expect elections to be called by next year.

In Washington, State Department spokeswoman Victoria Nuland said Friday that the United States has been communicating to Malaysia the importance of respecting human rights, including freedom of expression and assembly.

— Associated Press