KUALA LUMPUR, MALAYSIA - Malaysian police fired tear gas and water cannons, and arrested dozens of demonstrators Saturday in an attempt to disperse a mass street protest against draconian internal security laws.

Thousands of police backed by riot squad officers and helicopters cracked down at three rallying points in Kuala Lumpur - two major mosques and a popular shopping district.

Online news portal Malaysiakini said some 200 people had been arrested as police tried to disperse crowds at the rallying points and scupper their plans to march to the royal palace or the city's Independence Square.

At least 50 rounds of tear gas were fired and water cannons were directed at a crowd of around 10,000 people who gathered at the Sogo shopping complex in downtown Kuala Lumpur.

An AFP reporter saw at least 25 people arrested there before the huge group began marching down a main thoroughfare towards the royal palace, triggering the police offensive.

Organisers said they intended to present a 10-point memo to the king including demands for the abolition of the Internal Security Act, which allows for detention without trial.

They are also calling for the closure of a camp in northern Perak state where detainees are held, and an inquiry into all deaths in custody and allegations of police abuse of power.

At the national mosque, where an AFP reporter saw at least 50 detained, opposition legislator Siti Mariah Mahmud, from the Islamic PAS party, criticised the arrests of protesters as they attempted to enter the mosque.

"This is not reasonable. It's prayer time and this action is a breach of our religious freedom and duty," she said.

When prayers were completed, those who had managed to enter the mosque streamed out, joining a crowd of at least 5,000 which began marching towards Independence Square before being confronted by tear gas and water cannons.

Prime Minister Najib Razak has denounced the protest plans, saying he had already agreed to review the ISA after coming to power earlier this year.

The legislation, a hangover from colonial days, has been used against government opponents as well as suspected Islamist terrorists.

"We are here to fight for the ISA to be abolished," said Yati Ali, 45, one of some 100 women and children standing in a group outside the Sogo complex.

"We don't fear arrest. ISA is a cruel act, we are fighting for justice," she said.