Demonstrators gathered in the Malaysian capital, Kuala Lumpur, to protest a security law that allows detention without trial, were dispersed by police using water cannons and tear gas.
The police estimated that about 10,000 protesters participated, while witnesses estimated that as many as 20,000 demonstrators took to the streets for the protest against Malaysia's Internal Security Act (ISA) on Saturday.
Organisers said they intended to present a 10-point memo to the king, including demands for the abolition of ISA and the closure of a camp in northern Perak state where detainees are held.
More than 60 of the 589 people detained in Saturday's protest, which saw at least 15,000 people massing in chaotic scenes in downtown Kuala Lumpur, were still in custody Sunday according to media reports and lawyers.
Ismail Omar, Malaysia's deputy inspector general of police, told Al Jazeera on Sunday that many would likely be charged with holding an "illegal procession" or with other offenses.
"Some of them without documents will be charged under the registration act and we're also looking under the societies act," he said.
He reportedly said protest organizers, including opposition leader Anwar Ibrahim, would also be questioned.
If those remaining in custody are charged with illegal assembly, they could face up to a year in prison and a fine if convicted.
Government authorities had earlier warned that they would not grant a permit for the protest, saying it could undermine public peace.
Human rights activists have held numerous smaller protests over the years against the security act, but Saturday's event received a boost after opposition parties urged their supporters to come out in force.
Government leaders have dismissed the protest as an attempt by the opposition to gain political mileage. Najib said the protest was "unnecessary and only caused hardship to the people" as the government had already pledged to review the controversial law.
But analysts said the police action was a step back for Najib, who has been battling efforts by opposition parties to portray him as a leader who disregards public opinion on issues such as human rights and freedom of expression. He has been struggling since taking office in April to revive his government's popularity amid public dissatisfaction over economic mismanagement and racial tensions.
The ISA was originally instituted under Britain's colonial rule of Malaysia.
AFP, Malaysia criticised over protest crackdown, 2 August 2009, http://www.google.com/hostednews/afp/article/ALeqM5iWadDiT-O2m3l0OFNda1l...
Al Jazeera, Scores held after Malaysia protest , 2 August 2009, http://english.aljazeera.net/news/asia-pacific/2009/08/2009824192616951....
The New York Times, Hundreds Held in Large Protest in Malaysia, 1 August 2009, http://www.nytimes.com/2009/08/02/world/asia/02malaysia.html
AP, Dozens in lockup after Malaysia police crush rally, 2 August 2009, http://www.google.com/hostednews/ap/article/ALeqM5jMkEt58qYIeNndlhOAFZQd...