Malaysia's ex-prime minister Najib Razak played a pivotal role in plundering the state's 1MDB fund and channelled hundreds of millions of dollars into his own bank account, a prosecutor said Wednesday at the opening of his most significant graft trial.

Claims that Najib and his cronies pilfered massive sums from the fund and spent it on everything from real estate to artworks contributed to the defeat of his long-ruling coalition to a reformist alliance.

After losing power last year, the former prime minister was arrested and hit with dozens of charges related to 1MDB. He is expected to face several trials. 

The first started in April but Wednesday's is the biggest, with Najib accused of 21 counts of money-laundering and four of abuse of power, centering on allegations that he illicitly obtained 2.28 billion ringgit ($540 million) from 1MDB. 

Opening the trial, lead prosecutor Gopal Sri Ram told the High Court in Kuala Lumpur that Najib was a key player in the plunder of 1Malaysia Development Berhad, a fund established with the ostensible aim of boosting Malaysia's economy.

"His objective was to enrich himself," the prosecutor told the packed courtroom, as he outlined a series of complex financial transactions between 2011 and 2014 he said were used to funnel cash to Najib. 

The 66-year-old, who was in court for the start of the trial, denies all the charges.