Thousands of anti-government protesters marched in Malaysia’s capital demanding the resignation of Prime Minister Najib Razak over his alleged involvement in a multibillion-dollar corruption scandal.

Clad in yellow shirts and unfazed by arrests of activists and opposition leaders, protesters marched on Saturday from various spots towards downtown Lumpur amid tight security.

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 centre of investigations in the United States and several other countries.

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

The US government complaints also said that more than $700m had landed in the accounts of “Malaysian Official 1”. They did not name the official, but appear to be referring to Najib.

Some were chanting “Save Democracy” and “Bersih, Bersih” – the name of the electoral reform group that organised the rally. The name means “clean” in the Malay language.

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Bersih said on Twitter that police raided its office on Friday and arrested its chairwoman Maria Chin and another official Mandeep Singh.

It said at least seven others including several politicians were also held. It said police confiscated laptops, mobile phones and bank statements.

“Despite the authorities’ desperate measures to stop us, [the rally] will go on,” the group tweeted.

Another Bersih official, Mandeep Singh, and 12 others including several politicians were also arrested, mostly in connection with the rally and to prevent rioting, the police said.

“The reason why people are on the streets is not just about asking him [Najib] to resign, but it is also about changing the system,” Azmi Sharom, a law professor at the University of Malaysia, told Al Jazeera.

“The ruling party can win less popular votes and yet win the large majority in the house, so it’s a serious systematic problem that needs to be changed and this is why most of the people are out on streets today.”

‘Heavy-handed’

The protesters gathered around the Independent Square, the main venue that was locked down by police. A smaller group of red-shirt pro-government supporters held a counter-rally.

Najib, who has denied any wrongdoing, 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,” said Wan Aisyah Wan Ariffin, an opposition supporter.

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A rally that Bersih organised in August 2015 demanding Najib’s resignation brought together 50,000 people, according to police estimates. Bersih said the number was much higher.

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

“Can protests change anything? We don’t know because that would require a huge move on the part of the election commission, it would need a parliament to agree on something, so it’s a bit of a long shot,” Sharom told Al Jazeera.

“But I think people are angry, they feel the desire, the need to express themselves and hopefully this would push and gather momentum and perhaps in the next elections something positive can come out of it.”