Najib, 66, faces a total of 42 charges related to the embezzlement of millions of dollars from the 1Malaysia Development Berhard (1MDB) fund that ended up in his private bank accounts, a scandal that caused the politician's downfall in the general elections held in May.
In the trial that started Wednesday - the most significant of the court proceedings in this case so far - prosecutors charged Najib with 21 counts of money laundering and four counts of abuse of power for his alleged embezzlement of 2.28 billion ringgit (around $546 million) from the 1MDB fund.
In his opening statement, prosecutor Gopal Sri Ram said that Najib used his triple status as head of government, Finance Minister and President of the 1MDB advisory board to exert influence over the fund's management and carry out "certain abnormal transactions with undue haste", Efe news reported.
"The ultimate aim of the accused was to obtain gratification for himself," the prosecutor said. "He succeeded in achieving that aim."
The trial started one day after the conclusion of the first lawsuit against Najib, which involved the embezzlement of 42 million ringgit ($10 million) from a 1MDB affiliate, for which he has been charged with seven counts of money laundering and abuse of power.
The judge is set to rule in November whether the prosecution's case is admitted - after which the defence would present its arguments - or it is dismissed, thus exonerating the former leader.
Najib created 1MDB in 2009, the year he rose to power, with the ostensible purpose of attracting foreign investment and creating a financial district in the country's capital, Kuala Lumpur.
However, the fund ended up accumulating a debt of 42 billion ringgit.
A change in government prompted the investigation into this scandal, which was uncovered in 2015 by The Wall Street Journal and the London-based online investigative journalism portal Sarawak Report.
Najib's wife, as well as several businessmen and bank executives close to the former Premier, have also been caught up in the scandal and face criminal charges.
The US Justice Department had estimated that the total amount of money embezzled from the fund reached $4.5 billion, of which around $1 billion were laundered inside the country through the purchase of luxury real estate, yachts, jewellery and artworks.
( With inputs from IANS )