Defying government warnings, around 15,000 protestors were on streets in response to the call by the opposition, and later a police spokesman said 175 people had been arrested amidst chaotic scenes in downtown area of Kuala Lumpur.
It all began in small gatherings in street corners which later swelled into a huge protest march with people intending to march to the Royal Palace to register their protest.
Crowds played cat-and-mouse with the police, disappearing into side streets at the first sign of heavy deployment only to re-emerge from different sides.
The protest caused huge traffic snarls in the city with buses going off the roads and the trains halted half-way to their destinations wary of stone pelting.
The new Prime Minister Najib Tun Razak denounced the protest saying it was unnecessary as he had already announced to review the Internal Security Act, a hangover from colonial times.
The gathering is being considered the largest since the 2007 ethnic Indian protests organised by the outlawed Hindu Rights Action Force. Some online sites put the number of demonstrators as high as 20,000.
People from early morning began gathering at city's main mosque, shopping malls and a railway station and started marching towards the national palace. They chanted "Reformasi", the opposition political party's slogan for political change, and tried to submit a petition to the country's king to denounce the act.
They also called for the closures 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.
Police backed by riot squad officers and helicopters scuffled with protestors and fired tear gas and chemical-laced water to disperse them. They also charged the protestors with batons.
"The police are really brutal," opposition leader Anwar Ibrahim told mediapersons at the protest. "This clearly shows (the government's) intolerance to any dissent. ... We gather here today to fight a cruel law."
Some online reports claimed that around 200 people had been arrested and 75 rounds of tear gas were fired.
Among the detained were several top political figures including R Sivarasa, vice-president of opposition leader Anwar Ibrahim's party. The sustained police offensive sent the huge crowd scattering, with many coughing and choking from the effects of the gas.
The roadblocks set up by the authorities to stop the demonstrators from entering downtown area resulted in massive traffic jams around the city and adjacent areas. Restaurants and stores were closed in some places ahead of the planned rally.
Bernama news agency also reported Higher Education Minister Mohamed Khaled Nordin as saying that legal action would be taken against tertiary students found participating in anti or pro-ISA gatherings.
"If there are students involved in the gatherings and if they allow themselves to be victimised and used for other people's agenda, I will not protect them," he told mediapersons.
