A day after a mass anti-government rally it organized, the Malaysian electoral reform group Bersih voiced concern Sunday over the arrest of its chairwoman under a security law meant for terrorists and the detention of more than a dozen people.

Police say 15,500 protesters took to Kuala Lumpur's streets on Saturday to demand the resignation of Prime Minister Najib Razak over a financial scandal. Bersih estimated the crowd at 120,000.

Bersih official Wong Chin Huat said the security sweep and chairwoman Maria Chin's detention without trial under the Security Offences Act were vindictive and aimed at silencing dissent.

"The crackdown shows that Najib is feeling insecure. It is a sign of panic," Wong told The Associated Press on Sunday. "But the people have sent a clear message. They want reforms and they want Najib to step down."

Police raided Bersih's office on Friday and took Chin in for investigations into "activities detrimental to parliamentary democracy." Chin was formally detained Saturday under the security law and can be held without trial for 28 days.

Bersih said on Facebook that its lawyers finally met Chin on Sunday after having no access to her since she was detained.

The group said Chin was being held in solitary confinement in a small cell with no windows and two small lightbulbs. It said Chin was not given any bed or pillow, and had just the cement floor and a slab of wood.

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One of Bersih's lawyers, Fadiah Nadwa Fikri, said by phone that the law has been abused to punish Chin. If convicted, she said Chin faces up to 15 years in jail.

Apart from Chin, Fadiah said 15 other politicians and activists, including three Bersih officials, have also been detained.

Prime Minister Najib, who is attending an Asia-Pacific summit in Peru, was quoted by Malaysian national news agency Bernama as saying that Saturday's rally calling for him to step down was unconstitutional and that Malaysians were fed up with demonstrations.

"We, Malaysians, must uphold the principle of the rule of law. Otherwise there will be chaos in the country and the people will suffer," said Najib, who earlier had slammed Bersih rallies as a tool for the opposition to unseat the government.

An August 2015 Bersih rally that also demanded that Najib quit drew around 50,000 demonstrators.

Amnesty International has called for the immediate release of the activists, describing them as prisoners of conscience. Instead of investigating allegations of corruption and rights abuses, the government is "silencing people for raising these concerns," the group said.

Najib has kept an iron grip since graft allegations emerged two years ago involving the indebted 1MDB state fund. He founded the fund in 2009 to promote economic development, but it accumulated billions in debts over the years.

The investigations into 1MDB in the U.S. and several other countries are centered on allegations of a global embezzlement and money-laundering scheme.

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.