Malaysian protestors unfold a banner reading 'BERSIH 4' while marching through the city streets during a BERSIH (The Coalition for Free and Fair Elections) rally in Kuala Lumpur, Malaysia, August 29, 2015. [Photo/IC]

Former Prime Minister Mahathir Mohamad, who has been spearheading calls for Najib's resignation, added momentum to the rally when he made a surprise brief appearance late Saturday with his wife to loud cheers from the crowd, and telling protesters to "carry on."

Najib has been fighting for political survival after leaked documents in July showed he received some $700 million in his private accounts from entities linked to indebted state fund 1MDB. He later said the money was a donation from the Middle East, fired his critical deputy and four other Cabinet members as well as the attorney general investigating him.

He slammed the protests for tarnishing Malaysia's image.

"Those who wear this yellow attire ... they want to discredit our good name, scribble black coal on Malaysia's face to the outside world," Najib was quoted as saying by national news agency Bernama.