Police in Malaysia used tear gas and water cannons to disperse more than 10,000 demonstrators who gathered in the capital, Kuala Lumpur, to protest a law allowing detention without trial.
Local police said close to 600 people, including women and young people under 18, were arrested Saturday outside the national mosque and shopping mall.
Authorities set up roadblocks and introduced other security measures to curb rallies organized by supporters and opponents of the country's Internal Security Act.
Malaysian authorities banned both demonstrations, saying they would undermine public peace.
Under the law, people considered national security threats can be detained for long periods of time without trials.
Prime Minister Najib Razak urged demonstrators to refrain from rallying, saying the law will be reviewed.
In his first act after taking office in May, the prime minister freed 13 people who had been detained under the security act. But a number of people, most of them suspected of links to terrorist groups such as Jemaah Islamiyah, are still detained under the act.
Critics say the law is often used to detain outspoken government opponents. They have also condemned the secrecy and alleged torture linked to the use of the law. But some government officials say it is necessary for national security.
Some information for this report was provided by AFP, AP and Reuters.