Bersih, Malaysia's leading alliance of independent NGOs, rights groups and reform advocates, vows to bring out tens of thousands of people in the capital Kuala Lumpur and two other cities on Saturday and Sunday.
Bersih slams MCMC ban on rally information http://t.co/Zu0stLIelY
— Malaysian Insider (@tm_insider) August 27, 2015
Authorities called the rally "illegal," raising fears of violent clashes.
The government will block websites that "promote, spread information, and encourage people to join" the demonstration, the Malaysian Communications and Multimedia Commission (MCMC) said in a statement.
It said the rally "threatens stability" and will "tarnish the country's image". It was not clear how widely the blocks would be applied.
Najib's cabinet ministers have admitted he received nearly $700 million in mysterious deposits into his personal bank accounts starting in 2013, a revelation first brought to light by a Wall Street Journal investigative report last month.
Najib had already been under pressure over months of allegations that huge sums had disappeared from deals involving heavily indebted state investment company 1Malaysia Development Berhad (1MDB), which Najib launched in 2009.
Read: Malaysian PM reshuffles cabinet, dumps deputy after 1MDB criticism
Najib and 1MDB vehemently deny wrongdoing.
Cabinet ministers say the money transfers were "political donations" from unidentified Middle Eastern sources, and that there was nothing improper. No further details have been given.
The accounts have since been closed and the whereabouts of the money is unknown.
The revelation has provoked outrage among many Malaysians, including members of Najib's ruling party.
Authorities already have slapped a three-month suspension on a newspaper that reported aggressively on the scandal, and have warned that anyone highlighting the issue could face charges of attempting to subvert the government.
Najib's party has controlled Malaysia since independence from Britain in 1957, but has seen voter support slide in recent years over its authoritarian tactics and recurring corruption scandals.
Malaysian authorities typically take a tough line against anti-government protests.
Previous rallies by Bersih for electoral reform have ended in clashes with police, most recently in 2012.
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