Malaysia's top court on Thursday began hearing ex-leader Najib Razak's appeal to overturn his jail sentence for corruption in a high-stakes legal gambit that could see him locked up or potentially launching a political comeback.

Following a brief drama at the Federal Court seen as an attempt by defence lawyers to delay proceedings, government prosecutors started outlining why the former prime minister's 12-year jail sentence over a financial scandal at state fund 1MDB should stay.

The court dismissed Najib's plea for a retrial two days earlier, clearing the way for the hearings, which will be held until August 26.

Lead defence lawyer Hisyam Teh Poh Teik surprised the court Thursday by telling the panel of five judges that he wanted to be discharged from the case, citing inadequate time to prepare as he had been recently appointed.

The court, however, refused to discharge him and Chief Justice Tengku Maimun Tuan Mat ordered the prosecution to present its case.

Najib, 69, and his ruling party were roundly defeated in 2018 elections following allegations of their involvement in the multibillion-dollar scandal.

He and his associates were accused of stealing billions of dollars from the country's investment vehicle and spending it on everything from high-end real estate to pricey art.

Following a lengthy High Court trial, Najib was found guilty of abuse of power, money laundering and criminal breach of trust over the transfer of 42 million ringgit ($10.1 million) from former 1MDB unit SRC International to his personal bank account.

He was sentenced to 12 years in jail in July 2020, though he has not been sent to prison while the appeals process plays out.

An appellate court last December rejected his appeal, prompting him to mount a final plea before the Federal Court.

Najib had been hoping the court would grant a full retrial but that request was unanimously rejected on Tuesday.

- No easy comeback -

Dressed in a dark suit and white mask, Najib arrived in court and waved to about 70 supporters, who shouted "bossku", meaning "my boss", which has become a rallying cry among his defenders.

"This is not a fair trial. Najib is a victim of political persecution," one of the supporters, 65-year-old businessman Mohamad Yusof Khalid, told AFP.

But government prosecutor V. Sithambaram painted a different picture in court.

He said the funds that went into Najib's bank account "were used for his personal expenses like buying Chanel watches and to pay credit cards".

In a statement after the hearing, Najib objected "in the strongest of terms" to how the court forced his lawyer to continue despite him wanting to be discharged, which he said effectively left him without proper counsel.

"I am now in a situation where my right to counsel and a fair hearing is illusory," he said.

If the conviction is upheld, Najib will begin serving his jail sentence immediately, lawyers said.

An acquittal, however, could propel him into contention for his former political post, as he remains popular in Malaysia despite the scandal that plagued his administration.

He remains a lawmaker with the United Malays National Organisation (UMNO), the leading party in the current government, and helped its candidates win in recent by-elections.

Some analysts, however, said it may not be an easy comeback for Najib as rivals within his own party may try to derail him.

"Many UMNO leaders privately like to see him checking (into prison) to allow the party a fresh break," said political scientist Wong Chin Huat of Sunway University, Malaysia.

"With him gone there is one fewer competitor," added Oh Ei Sun, principal adviser for the think-tank Pacific Research Center of Malaysia.