Kuala Lumpur  - Malaysian police on Saturday detained hundreds of protestors taking part in a massive street rally against a draconian security law.

Tens of thousands gathered at three rallying points in the capital Kuala Lumpur, despite heavy police presence and road barricades around town since Friday.

Riot police fired chemical-laced water cannons and tear gas at no less than five different areas since early Saturday in attempts to disperse the swelling crowds, and prevent them from marching towards the city's Independence Square.

Official media reported that more than 150 people have been detained as of noon Saturday, but several opposition websites claimed that more than 200 have been arrested. None have reported injuries.

The protestors are calling for the abolishment of the Internal Security Act, a decades-old law that allows for indefinite detention without trial for those the government suspects of being a threat to national security.

However, critics say the act is being used by the government to stifle political dissent, a charge the government denies.

Prime Minister Najib Razak had urged organizers not to go ahead with the protest, saying he had already agreed to review the law.

On Saturday, Nazri Aziz, the minister in charge of law, maintained that the act would not be abolished, as long as the ruling coalition was in power.

"The ISA is a necessary law," Nazri was quoted as saying by the Star online news portal.

Some 20 people are believed to be currently detained under the Internal Security Act, mostly for charges of Islamic terrorism and forgery cases. (dpa)