The Malaysian Communications and Multimedia Commission (MCMC) said in a post on its official Facebook page on Thursday that it would block websites that "promote, spread information and encourage people to participate" in the protest organized by the civil society group Bersih.

It said this followed a statement earlier in the week by the home ministry that Bersih was spreading anti-government propaganda which had damaged the country's image and was a threat to its safety and economic stability.

Najib has been embroiled in a political storm amid allegations of graft and financial mismanagement at debt-laden state fund 1Malaysia Development Bhd (1MDB), whose advisory board he chairs.

The government has suspended two newspapers and a website reporting on 1MDB and earlier this month said it was seeking help from social media and Internet companies to "curb" content for public safety.

The Kuala Lumpur authorities rejected Bersih's application for a permit to hold its rally in the city, setting the stage for a possible showdown with security forces when protesters gather on Saturday at five venues and try to converge.

Police used water cannon and tear gas to disperse protesters at Bersih's last big rally in 2012.

Bersih plans to hold the demonstration from Saturday afternoon until the end of the evening on Sunday, the eve of Malaysia's independence day, in Kuala Lumpur and in the two Borneo cities of Kota Kinabalu and Kuching.