Malaysian student activists have rallied to demand the arrest of Prime Minister Najib Razak, who has been implicated in a U.S. government probe into a massive fraud in a Malaysian investment fund.

More than 1,000 people defied a police ban as they congregated at two locations in Kuala Lumpur on Saturday, holding posters and caricatures of Najib.

The U.S. Department of Justice said last month that at least $3.5 billion has been stolen from the Malaysian fund founded by Najib. It has initiated action to seize $1.3 billion it said was used to buy assets in the U.S.

It said in court filings that more than $700 million had landed in a Malaysian official's accounts. It didn't name the official, but appeared to be referring to Najib.