Kuala Lumpur: More than 50,000 people on Saturday poured onto the streets of Malaysian capital in the biggest political showdown in years to demand electoral reforms, defying a massive lockdown with authorities arresting more than 1,600 people, including opposition leaders.

People came out in large numbers for the opposition- backed rally to bring pressure on Prime Minister Najib Razak's long ruling coalition to usher in electoral reforms and more transparency ahead of national polls early next year.

Police fired tear gas and detained more than 1,600 people as they sealed off all major roads leading to the city centre in the capital, closed train stations and deployed trucks mounted with water cannons to thwart the rally.

Leaders of the opposition parties were among those detained in a massive security operation to stall the rally, which failed as more than 50,000 people took to the streets, organisers claimed.

Protesters faced baton wielding policemen in front of the downtown bus station as they attempted to march to independence stadium, the venue of the rally. Opposition leader Anwar Ibrahim was injured during the protest.

The rallyist broke into three groups with some attempting to march to the king's palace to handover a memorandum detailing their demands.

Among those arrested were Ambiga Sreenivasan and Maria Chin Abdullah, the two leaders of the Bersih, the opposition coalition that had organised the rally.

Thousands of other marchers were also trying to make their way to the venue, shouting slogans 'long live the people' and 'we want electoral reforms'. Police have declared the protest illegal, warning of chaos in the streets. Hundreds of policemen, many armed with batons and anti-riot gear and backed by water cannon, were deployed in strategic locations around the capital.

The rally marked the culmination of a campaign launched by opposition parties who have accused Najib's national front government of clinging to power for 54-years through electoral frauds.

The rally, called by the non-governmental organization Bersih 2.0 (meaning clean in Malay language), was to be held at the city centre but after Prime Minister Najib suggested that the march be held inside a stadium, Bersih chief Ambiga Sreenevasan agreed.

At least 151 women and 16 children were among the 1,667 people detained during the rally.

City police CID chief Ku Chin Wah said all the detainees were expected to be released later in the day.

Police fired numerous rounds of tear gas in an attempt to disperse the crowds. The demonstrators dispersed after a five-hour standoff with police.

Najib said later that the protesters only represent a minority, and that most Malaysians support his administration.

"If there are people who want to hold the illegal rally, there are even more who are against their plan," the prime minister was quoted as saying by the national news agency, Bernama.

Bersih wants and an overhaul of voter registration lists, tougher measures to curb fraud and fairer opportunities for opposition politicians to campaign in government-linked media.