Thousands of protesters have rallied in Malaysia's capital, calling for the resignation of Prime Minister Najib Razak over a corruption scandal.

The protesters marched from different spots towards downtown Kuala Lumpur amid tightened security measures on Saturday, chanting "Save Democracy" and "Bersih, Bersih," which means “clean” in Malaysian.

Najib is implicated in a multi-billion dollar misappropriation scandal, including about $700 million transferred to his private accounts by the Saudi royal family.

The protest came after the head of pro-democracy group Bersih was arrested on Friday, along with several opposition leaders and student activists.

"Our country is being governed by clowns and crooks. So I'm here to protest against our prime minister," said artist Fahmi Reza, carrying a poster of a clown-faced Najib.

"We want to see Malaysia more developed and not robbed of billions of ringgit," another protester, Wan Aisyah Wan Ariffin, said.

This is the second time in 15 months that huge crowds have turned out to vent anger over allegations that billions of dollars were looted from Najib's brainchild investment fund, 1MDB.

The scandal has gripped the country for more than a year and sparked investigations in several countries.

Lawsuits filed by the US Justice Department in July said over $3.5 billion was stolen from 1MDB, which was founded by Najib, and that some of those funds flowed into the accounts of an unnamed top Malaysian official. A Malaysian Cabinet official has since admitted that individual was Najib.

Najib last year abruptly shut down Malaysian investigations, fired the attorney general and shored up his position by purging ruling-party critics.

Judiciary authorities later cleared him of any criminal wrongdoing in the case, saying the money was from donations from the Saudi royal family. Critics have dismissed the ruling, arguing that the transfer of personal donations did not rule out corruption.

Police form a barricade to block pro-democracy group Bersih from marching during a protest in Kuala Lumpur, Nov. 19, 2016. (Photo by Reuters)

The Wall Street Journal said in early July that about $700 million had been transferred to Najib’s private accounts before the 2013 general elections.

Saudi Foreign Minister Adel al-Jubeir said in April that the $681 million Riyadh offered to the Malaysian premier was a “genuine donation with nothing expected in return.” Najib returned from a four-day visit to Saudi Arabia back in March.

Tensions in the Muslim-majority country have spiraled in recent weeks following threats by the "Red Shirts" ethnic-Malay rightist group to disrupt anti-government demonstrations.

Bersih, otherwise known as "Yellow Shirts", organized the Saturday protest. Ethnic Chinese and Indian minorities formed the bulk of a similar rally of more than 200,000 organized by Bersih last year.

Pro-government supporters also staged a counter-rally on Saturday amid fears of possible clashes between the rival protesters.

Najib, who is attending the Asia-Pacific Economic Cooperation (APEC) summit in Peru, accused Bersih of being a tool for opposition parties to unseat his government.

"Their movement is deceitful. It is clear that these street protests are in fact the opposition disguised as an independent NGO working to unseat a democratically-elected government," he said on his website on Friday.