Malaysia's prime minister Tuesday warned he could use a security law which allows for detention without trial to crack down on street protests after two mass rallies in the capital.

The controversial Internal Security Act (ISA), which rights groups are campaigning to have abolished, is mostly used against alleged Islamic militants.

"The ISA is a preventive measure to spare the nation from untoward incidents that can harm the prevailing peace and harmony and create all sorts of adverse things," Prime Minister Abdullah Ahmad Badawi said, according to the state Bernama news agency.

"So I don't know (when to invoke the ISA), but ISA will be there. When it is appropriate to use it, it will be used," he said.