A Malaysian court on Monday sentenced an opposition lawmaker to 18 months in prison for releasing confidential information related to scandal-hit state fund 1Malaysia Development Berhad (1MDB), keeping up a crackdown on critics of the government.

The verdict comes just days ahead of a massive rally planned on Saturday by pro-democracy group Bersih, calling for Prime Minister Najib Razak to step down over his alleged involvement in the 1MDB scandal.

Rafizi Ramli, a member of the People's Justice Party (PKR), was found guilty of violating the Official Secrets Act (OSA), by possessing and disclosing a page from the auditor-general's report on 1MDB without permission earlier this year.

The audit was carried out last year following allegations of mismanagement and graft at 1MDB and has yet to be declassified.

The court allowed a stay on the execution pending an appeal of the verdict. Rafizi said on his Twitter account that he would appeal the verdict.

‘This is a sad day for Malaysian democracy, when a democrat has been brought to court for daring to speak up to protect the rights and interest of the people,’ PKR chief Wan Azizah Wan Ismail said in a statement.

Wan Azizah's husband, charismatic opposition leader Anwar Ibrahim, is serving a five-year prison sentence after being found guilty in a case that supporters and many independent observers regarded as politically motivated.

He was convicted in 2015, just as as the pressure on Najib began to mount over the financial scandal at 1MDB.

Founded by Najib, who chaired its advisory board, 1MDB is currently the subject of money laundering investigations in at least six countries, including Switzerland, Singapore and the United States.

Najib has denied any wrongdoing.

The government has clamped down on anti-government rally organisers, opposition leaders, activists and media groups who have raised the 1MDB issue. It has also proposed amendments to the Malaysia media law and Legal Profession Act.

Phil Robertson of Human Rights Watch said in a statement that Rafizi's conviction under the OSA was ‘unprecedented’ and the act was used to ‘intimidate whistle-blowers into silence’.

Responding to the verdict, Minister in the Prime Minister's Department Abdul Rahman Dahlan said Rafizi had been repeatedly warned that he would be breaching the Official Secrets Act if he disclosed content from the audit.

‘He tried a cheap stunt for personal political gain but he knowingly committed a serious crime in doing so. It is right that he pays the price, and he only has himself to blame,’ he said in a statement.