Malaysia's former leader Najib Razak was hit with 25 new charges of money laundering and abuse of power linked to the 1MDB mega-scandal on Thursday, including allegations that hundreds of millions of dollars were transferred to his personal bank account.

Najib pleaded not guilty to all the charges during a court appearance in Kuala Lumpur. Since his shock election defeat in May, he has now been charged with a total of 32 counts related to state fund 1MDB, which he and his cronies allegedly looted.

He faced four abuse of power and 21 money-laundering charges Thursday, in what observers said represented the most serious allegations he has yet faced.

He was accused of receiving illegal transfers to his own accounts from the fund totalling nearly 2.3 billion ringgit ($555 million). The suspicious transfers were a key chapter in the scandal, and Najib had previously insisted they were a donation from Saudi royalty.

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Allegations that Najib and his cronies pilfered huge sums from the investment vehicle and spent them on everything from high-end real estate to artwork were a major factor in the defeat of his long-ruling coalition to a reformist alliance headed by Mahathir Mohamad.

Mahathir, 93, who is in his second stint as premier after coming out of retirement to take on scandal-mired Najib, has ramped up probes in the 1MDB scandal and vowed to have the ex-leader prosecuted.

Najib was initially detained on Wednesday by the Malaysian Anti-Corruption Commission over the alleged bank transfers, before facing further questioning by police on Thursday morning.

The raft of charges filed on Thursday accused Najib of receiving vast amounts of money in a complex series of transactions.

'Fight for the truth'

Najib, dressed in a dark blue suit, passed through a media scrum as he arrived at court, and a small groups of his supporters waved banners emblazoned with the logo "fight for the truth" and chanted "rise up people".

He had already been charged with money-laundering, criminal breach of trust and abusing his position over claims he pocketed some $10 million from a former unit of 1MDB. He denies any wrongdoing.

But the latest charges are more significant and relate to a key episode in the 1MDB scandal.

When reports surfaced in 2015 that huge sums from 1MDB flowed into Najib's bank accounts before a hotly contested election, it dramatically ratcheted up pressure on the leader and his inner circle over the scandal.

The attorney-general later cleared Najib of any wrongdoing, saying the money was a personal donation from the Saudi royal family, and closed down domestic investigations.

Read: Treasure trove worth $273m seized from ex-Malaysian PM

But as allegations surrounding the fund multiplied, Najib became increasingly authoritarian, jailing political opponents and introducing laws that critics said were aimed at stifling dissent.

1Malaysia Development Berhad was set up in 2009 and overseen by Najib, with the purported aim of promoting economic development in Malaysia, but accusations quickly mounted that the fund was a vehicle for the leader and his cronies to loot state coffers.

Najib and his luxury-loving wife Rosmah Mansor became hated figures, symbols of the rot in a corrupt elite that had ruled Malaysia uninterrupted since independence from Britain in 1957.

The US Department of Justice, which is seeking to recover assets allegedly bought with stolen 1MDB money, estimates that $4.5 billion in total was looted from the fund.