A Singaporean activist was jailed for three weeks Monday after pleading guilty to holding a small protest aboard a metro train against the controversial 1980s arrest of alleged communists.

The tightly controlled city-state has tough laws against dissent that make it illegal for even one person to hold a demonstration without a police permit.

Jolovan Wham, who has had numerous run-ins with authorities, and a group of fellow activists staged a protest aboard the train in June 2017.

They were demanding "justice" for a group of 22 people arrested in 1987 under tough laws that allow for detention without trial.

The group was accused of being part of an alleged communist plot to overthrow the government.

While the government insists the arrests were aimed at stopping a Marxist plot, critics say they were politically motivated.

Wham was also charged with vandalism for pasting notices on the train's window, and with refusing to sign a police statement related to the protest.

He pleaded guilty to all three charges, and was ordered to pay Sg$8,000 (US$6,049), or serve 32 days in jail. He chose to pay part of the fine and serve 22 days in jail.

The judge described Wham as a "repeat offender".

He served a 10-day jail sentence last year for organising an event in 2016 that featured prominent Hong Kong democracy campaigner Joshua Wong, speaking via Skype.

Wham, 40, told the court: "I am not ashamed of what I have done... I believe my conscience is still clear".

Phil Robertson, deputy Asia director at Human Rights Watch, criticised Singapore authorities for "pursuing an arbitrary, discriminatory, and vindictive campaign against" Wham.