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.

Shocking claims

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.

Protesters in Kuala Lumpur called for the resignation of Malaysian Prime Minister Najib Razak.

'Preposterous'

According to Shahrul, police confiscated computers and documents related to financial and money transactions, including the group's staff wages and payroll documents. Programs and brochures from overseas were also confiscated.

"There are people now claiming that Maria has links with foreign powers such as the CIA and what not. That is just preposterous," he said.

"SOSMA is to be used against terrorists and dangerous individuals. Maria is doing something good towards nation-building. Even if it is against the lines of the current government, there is no justification for this detention."

According to Chin's lawyers and family, she is currently being held in solitary confinement, in a windowless room with no bed or mattress, and is being denied access to medication.

Police did not respond to a CNN request for comment. Khalid told reporters that Chin is being handled like any other detainee and denied claims that she was being mistreated.

"We consider Chin's detention as arbitrary and the conditions that she is being held under is unacceptable," Shamini Darshni, executive director of Amnesty International Malaysia, told CNN.

"Solitary confinement is reserved for only the most hardened criminals and in Malaysia, it is usually imposed on prisoners who are on death row."

Darshni said that representatives of the Human Rights Commission of Malaysia had confirmed that Chin now has access to her medication and a doctor if required.

Shahrul Aman Mohd Saari, acting Bersih chairman, said that Chin's detention was "shocking."

'Maria's not a terrorist'

Regular Malaysians CNN spoke to were shocked by Chin's arrest. Murina Pawanteh, who works in public relations, said Bersih was a peaceful protest.

"I would like to hear from the authorities why Chin has been arrested in SOSMA and put in (solitary confinement)," she said.

"It's a very simple thing. If the government isn't transparent about it, then of course we will question."

Journalism student Bala Murugan said Chin "isn't a terrorist."

"She didn't cause terror and she didn't create panic," he said. "What she has done is actually to unite the people of Malaysia."

Steve Lam, a university lecturer, said that Chin's detention was "ridiculous" and called for the authorities to ensure that she was held under proper conditions.