More than 10,000 yellow-shirt protesters rallied Saturday in Kuala Lumpur seeking Malaysian Prime Minister Najib Razak's resignation over a financial scandal, undeterred by a police ban and the arrest of more than a dozen activists.

Police barricaded key roads in downtown Kuala Lumpur with water-cannon trucks on standby, but it did not stop the protesters. Some chanted "Save Democracy" and "Bersih, Bersih" — the name of the electoral reform group that organized the rally. The name means "clean" in the Malay language.

Former Prime Minister Mahathir Mohamad, who has been spearheading calls for Najib's resignation, joined the rally, adding momentum to the demonstration.

"Everybody feels concerned about the kind of government we have now," said Mahathir, wearing a yellow Bersih shirt. "The government is very cowardly, trying to prevent a demonstration, which is the right of the people."

Najib, who is attending an Asia-Pacific summit in Lima, Peru, has kept an iron grip since corruption allegations emerged two years ago involving the indebted 1MDB state fund that he founded. 1MDB is at the center of investigations in the U.S. and several other countries.

Najib, who has denied any wrongdoing, has said he won't be cowed by the rallies.

In a statement on his blog, Najib called Bersih "deceitful" and said the group has become a tool for opposition parties to unseat a democratically elected government.

"We want to see Malaysia more developed and not robbed of billions of ringgit," singer Wan Aishah Wan Ariffin, an opposition supporter, said at the rally.

Bersih later told its supporters to move the rally outside the Kuala Lumpur City Center, one of the world's tallest twin towers, with police locking down Independent Square, where the protesters had been trying to gather. A smaller group of red-shirt pro-government supporters held a counter-rally in the city.

Police said in a statement that they raided the Bersih office on Friday and detained its chairwoman, Maria Chin, for investigation into "activities detrimental to parliamentary democracy."

Thirteen other people, mostly politicians and activists, were also held to prevent rioting, police said.

Those detained included ruling party politician Jamal Mohamad Yunos, whose supporters trooped to downtown to counter the Bersih rally. Police have banned both events by Bersih's yellow-shirt supporters and Jamal's red-shirt group.

Bersih said on Twitter that three other activists were nabbed Saturday.

It later said Chin was formally detained Saturday under a security law and can be held for a further 28 days. The other activists were remanded for several days in police custody.

This marked the fifth rally organized by Bersih, which also held similar protests concurrently in two Malaysian cities on Borneo island. The last rally that Bersih organized in August 2015 also demanded Najib's resignation, attracting a crowd of 50,000 people, according to police estimates. Bersih said the number was much higher.

Human rights group Amnesty International slammed the crackdown and called for the immediate release of the Bersih activists, describing them as prisoners of conscience.

"These arrests are the latest in a series of crude and heavy-handed attempts to intimidate Malaysian civil society activists and other human rights defenders," Amnesty said in a statement.

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 U.S. Justice Department said that at least $3.5 billion had been stolen from 1MDB by people close to Najib and initiated action in July to seize $1.3 billion it said was taken from the fund to buy assets in the U.S.

The U.S. government complaints also said that more than $700 million had landed in the accounts of "Malaysian Official 1." They did not name the official, but appear to be referring to Najib. Support for Najib's National Front has eroded in the last two general elections. It won in 2013, but lost the popular vote for the first time to an opposition alliance.