Kuala Lumpur (CNN) – The arrest of a prominent Malaysian civil rights leader under a controversial anti-extremism law has sparked outrage at the government of embattled Prime Minister Najib Razak.

Maria Chin Abdullah was arrested on Friday on the eve of a mass rally organized by Bersih, a coalition of pro-democracy groups fighting for clean and fair elections.

Thousands of Malaysians took to the streets of the country’s capital, Kuala Lumpur, on Saturday to call for Najib’s resignation.

Under the Security Offenses (Special Measures) Act 2012, or SOSMA, people can be detained without trail for up to 28 days.

“I can’t say that we were surprised about the detention,” said Shahrul Aman Mohd Saari, acting Bersih chairman. “But we were really shocked that it came under SOSMA.”

Khalid Abu Bakar, Malaysia’s Inspector-General of Police, told reporters this week that Chin’s detention was unrelated to Saturday’s rally.

He said that during a police raid of Bersih’s premises, they found “documents detrimental to parliamentary democracy.”

Shahrul said that during the raid, “police went through every single item and document and they kept mentioning ‘OSF,’ which refers to the Open Society Foundation.

OSF is a non-profit organization founded by businessman George Soros that funds civil society groups around the world.

[Read the full story at CNN.com]