The group, which is supported by many civil groups and opposition parties, has vowed to proceed with tomorrow's rally in Kuala Lumpur despite a police ban and fears of clashes with a pro-government group.
Bersih said on Twitter that its chairwoman Maria Chin and another official, Mandeep Singh, were taken by police. It said police also confiscated 10 laptops, payrolls and bank statements.
Police officials could not be reached for immediate comment.
Najib has kept an iron grip since graft allegations emerged two years ago in the indebted 1MDB state fund that he founded. 1MDB is at the center of investigations in the U.S. and several other countries.
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A rally that Bersih organised in August 2015 also demanding Najib's resignation brought together 50,000 people, according to police estimates. Bersih said the number was much higher.
Najib, who has denied any wrongdoing, has said he won't be cowed by the rallies.
A ruling party politician, Jamal Mohamad Yunos, plans to lead a counter rally in Kuala Lumpur on Saturday, prompting fears of a clash between his group, dubbed the "Red Shirts," and Bersih's "Yellow Shirt" supporters.
Police have banned both rallies and the government has warned that police will act against any participants.
The investigations into 1MDB fund are centered on allegations of a global embezzlement and money-laundering scheme. Najib started the fund shortly after taking office in 2009 to promote economic development projects, but the fund accumulated billions in debt over the years.
The US government complaints also said that more than USD 700 million had landed in the accounts of "Malaysian Official 1." They did not name the official, but appear to be referring to Najib.
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