Malaysia’s former prime minister Mahathir Mohamad, 91, faces possible arrest following a huge anti-government rally in which he called on protesters to topple Prime Minister Najib Razak.

Mohamad accused Najib of “abusing the law” and “stealing the people’s money” as he addressed tens of thousands of people who had joined street protests at the weekend to demand Najib step down over corruption allegations.

Scores of activists, opposition and government politicians were arrested before the rally, in a move some observers said was a “crackdown” aimed at stifling dissent, but others said was necessary to prevent clashes between followers of Bersih, the electoral reform group that organised the rally, and the pro-government “red shirts”.

The chairwoman of Bersih, Maria Chin Abdullah, was arrested a day before the rally.

After the rally, Deputy Home Minister Nur Jazlan Mohamed warned that Mahathir could be arrested if he continued to incite “hatred towards the government”, but said authorities were cautious as the former prime minister might be trying to get arrested to gain public sympathy.

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“We know Dr Mahathir wants to get arrested but we will not fall into his trap,” Jazlan was quoted as saying by the government-owned Utusan Melayu.

Mahathir, who was prime minister from 1981-2003, is Malaysia’s longest serving leader. His son, Mukhriz Mahathir, told This Week in Asia: “I do not discount the government from doing something crazy without any care for what the people think about all this action.

“It would not be beneath [the government] to arrest even a former prime minister. If they were to arrest my father, reaction would come from all walks of life within Malaysia,” added Mukhriz, deputy president of his father’s new party, Malaysia United Pribumi Party.

Mukhriz was ousted as chief minister of the northern state of Kedah earlier this year after questioning Najib over the debt-riddled, scandal-hit state investment firm 1 Malaysia Development Berhad, or 1MDB.

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Najib was the chairman of 1MDB’s advisory board when the firm racked up RM42 billion of debt in five years.

Najib’s former political secretary, Oh Ei Sun, dismissed the possibility of a Mahathir arrest.

“No, that would tip the balance and create sympathy and anger among the fence-sitters. A very delicate balance,” said Oh, who served as Najib’s press secretary from 2009-2011.

In the run-up to the Bersih rally, Jamal Yunos, a leader from the ruling Umno party and the de facto leader of the red shirts, had vowed to fight Bersih with blood.

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“I would interpret these arrests more as a precautionary action to prevent clashes between Bersih and the red shirts. As you can see, most of those who were arrested have since been released, except for Maria,” said Kadir Jasin, a close associate of Mahathir.

Jamal was also arrested before the rally.

Chin is held under the Security Offences Special Measures Act (Sosma), an anti-terrorism law, in a move condemned by the opposition and human rights groups.

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“She is held in solitary confinement in a small cell where there are no windows and the lights are turned on 24 hours,” her lawyer Eric Paulsen told This Week in Asia.

“When she left the cell to meet her family and lawyers last Sunday, she had to put on darkened goggles. She doesn’t know where she is held and neither do we,” said Paulsen.

A writ of habeas corpus was filed for her release on Tuesday.

“Sosma came into being to address terrorism. I don’t see how Maria organising Bersih constitutes a threat to national security,” Mukhriz said.

Mohd Puad Zarkashi, from the Supreme Council of Umno, defended Chin’s detention under Sosma as “reasonable and proper”.

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Puad cited Section III and IV of Sosma – threatening the safety of the public and the federation, and pursuing political change through illegal methods.

“As such, it should be investigated under Sosma,” said Puad.