Malaysian police fired tear gas and water cannon, making 1,400 arrests on Saturday during clashes with protesters who defied government warnings to rally in the capital for electoral reform.

Leaders of opposition parties were among those detained during a massive security operation but it failed to thwart the outlawed demonstration which saw 50,000 citizens take to the streets of Kuala Lumpur, according to organisers.

Protesters faced baton-wielding riot officers in front of a downtown bus station, retreating at times and regrouping to push back police lines in a cat-and-mouse confrontation that took place in a downpour.

Some demonstrators fought back by picking up tear gas canisters which they lobbed at police, AFP reporters said.

Opposition leader Anwar Ibrahim was injured during the protest when he fell onto the pavement after a tear gas attack and was taken to a hospital as he was feeling unwell, an aide said.

The protesters dissolved into three main groups, and by late afternoon all were trying to force their way through a tight police cordon to a stadium and then to the king's palace to hand over a memorandum detailing their demands.

The police line, however, held firm.

"Why is the government trying to intimidate citizens?" said Mohamad Manij Abdullah, 50, a businessman who joined the rally.

"We are only trying to reform elections and have a free and fair government," he told AFP.

National police chief Ismail Omar told a news conference police had detained 1,401 people and were investigating them for illegal assembly although many were expected to be released on bail.

Ismail said barricades around the capital Kuala Lumpur which had turned it into a ghost town since midnight would be dismantled if there were no further incidents.

Among those arrested were protest leader Ambiga Sreenivasan and Maria Chin Abdullah. Ambiga told AFP she was freed later Saturday without being charged.

Abdul Hadi Awang, president of the Pan-Malaysia Islamic Party (PAS), the country's largest Islamic opposition grouping, was also arrested.

The protesters rallied in several areas of the city but later began to disperse, said Subramaniam Pillay, one of the organisers, who described the day as "a great success".

Student Chew Ai Nee, 30, said: "We have to take to the streets because we have not been given any opportunity to express our demands for change... the government cannot silence us when we march."

Many of the protesters were shouting "Reformasi!" (Reforms), "God is great" and "Long Live the People."

However, Mukhriz Mahathir, a leading member of the powerful United Malays National Organisation (UMNO), told AFP the government had to act to prevent anarchy.

"We cannot allow a minority group to protest and stir trouble in the country," he said, accusing protesters of provoking the police into firing tear gas "so that they can accuse the government of being heavy handed."

UMNO is the dominant party in the ruling Barisan Nasional coalition led by Prime Minister Najib Razak.

New York-based watchdog Human Rights Watch (HRW) denounced the arrests.

"This is a maelstrom of the Malaysian authorities? own making," said Phil Robertson, deputy director for HRW's Asia Division.

Downtown Kuala Lumpur, normally a hive of activity on weekends, was deserted as major roads into the commercial and tourist district were sealed off.

Meanwhile, about 30 Malaysians living in South Korea rallied in Seoul in support, with another 80 marching through central Hong Kong.

Organisers had called for solidarity walks and demonstrations in countries including Australia, Cambodia, Japan, the United States and Taiwan.

The demonstrators want reforms, including the eradication of vote buying and the prevention of irregularities such as people illegally voting several times during elections.

Bersih, which organised the protest, wants to see the use of indelible ink to prevent multiple voting, equal access to the media for all parties and the cleaning-up of electoral rolls.

Malaysia's opposition made major gains in 2008 elections against the ruling coalition but said they could have won more if voting rules were fair.

The country's next elections are widely expected to be called early next year, with the opposition aiming to end Barisan's half century rule.