KUALA LUMPUR, Malaysia — More than 60 people were in custody today after police put down Malaysia's biggest street protest in nearly two years, firing tear gas and chemical-laced water at thousands demanding an end to a decades-old security law.

Witnesses estimated as many as 20,000 demonstrators took to the streets of Kuala Lumpur yesterday in defiance of government warnings for people to shun the rally against the Internal Security Act, which allows the indefinite imprisonment of people regarded as security threats.

The clampdown on the protest could damage support for Prime Minister Najib Razak, who took power in April and 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.

Kuala Lumpur police Chief Muhammad Sabtu Osman said authorities arrested 589 people after about six hours, during which riot police wielding batons chased protesters down, scuffled with them, and dragged many into detention trucks.

Muhammad Sabtu said 63 people remained in custody today and were being investigated for illegal assembly — an offense punishable by up to a year in prison and a fine. He did not say whether they would be charged.

Salahuddin Ayub, vice president of the opposition Pan-Malaysian Islamic Party, said those still held were expected to be brought to court later today in an effort to hold them longer.

The protesters, some wearing opposition T-shirts and headbands, began massing at Kuala Lumpur's main mosque, a shopping mall and a train station yesterday morning.

Police repeatedly fired volleys of tear gas and water laced with stinging chemicals to disperse the crowds after they began marching toward the national palace. The protesters — who chanted "Reformasi," the opposition's slogan for political change — wanted to submit a petition to the country's constitutional monarch denouncing the security act.

"The police are really brutal," opposition leader Anwar Ibrahim said at the protest.

"This clearly shows Najib's intolerance to any dissent."

Government authorities had warned they would not permit the protest, saying it could undermine public peace.

Salahuddin said several activists hoped to present the petition calling for the act's abolishment at the palace tomorrow.

He said the opposition still had not decided whether to organize more rallies.