Thousands of police officers have been deployed in the Malaysian capital ahead of a planned demonstration calling for the immediate resignation of embattled Prime Minister Najib Razak.

Officers are visible throughout Kuala Lumpur as the country's police chief insists that the demonstration would be illegal.

Authorities had rejected the protest organisers' permit application.

The protest, led by a coalition advocating electoral reform and good governance, is expected to draw tens of thousands of people to demand Najib's resignation over hundreds of millions of dollars in his accounts, which he claimed came from anonymous donors.