Malaysian police questioned Friday former premier Mahathir Mohamad, who is under investigation after calling on the current prime minister to step down over a financial scandal.

Mahathir, Malaysia's longest-serving leader who remains a highly respected and influential figure, has become the fiercest critic of Prime Minister Najib Razak, who is facing pressure over a graft scandal surrounding state fund 1Malaysia Development Berhad (1MDB).

In July, the Wall Street Journal reported that investigators looking into the indebted fund had found that nearly $700 million had been transferred into Najib's bank account.

Najib denies wrongdoing or taking any money for personal gain and the state anti-corruption agency said the money was a political donation from an unidentified Middle East benefactor.

But that has not stopped the criticism.

Mahathir, 90, made a surprise appearance in late August at an anti-government protest in Kuala Lumpur and called for a "people's power" movement to topple Najib.

National police chief Khalid Abu Bakar said soon afterwards that the police would take a statement from Mahathir on his comments and they visited his office on Friday to do so, his aide, Sufi Yusof, said.

Yusof declined give any details about the meeting or say what it had centered on.

The main organizer of the August rally, Maria Chin, was charged this month under the Peaceful Assembly Act for failing to notify authorities about the gathering. She has denied the charges.

Najib, who is chairman of 1MDB's advisory board, has hit back at dissenters within his party and the government. He sacked his deputy and other ministers who had publicly questioned him and also replaced the attorney-general who was investigating 1MDB.

Authorities also suspended two newspapers and blocked access to a website that had reported on 1MDB. Legal action was also taken against some opposition politicians and activists.