SINGAPORE (Reuters) - Singapore police said on Monday they were investigating a protest by several blindfolded activists who held up books on a subway train in a call for justice for 22 people detained in 1987 under a tough internal security law.

The activists held copies of a recently published book “1987: Singapore’s Marxist Conspiracy 30 Years On” on Saturday while sitting on a train in a rare protest in the tightly controlled city-state.

“The police confirm that a report has been lodged and are looking into the matter. Anyone with information can submit it online,” police said in a emailed statement.

Under strict public assembly laws, protests are only allowed at a designated downtown square.

Twenty-two people were detained in 1987, some for up to three years without trial, under the colonial-era Internal Security Act (ISA) for their alleged involvement in a Marxist conspiracy to overthrow the government.

One of the activists who took part in the train protest, and appeared in photographs of it posted on social media with three colleagues, later said they were seeking justice.

“The government has never proven that those detained were involved in a conspiracy or were a threat to national security,” the activist, Jolovan Wham, told the news website, the Online Citizen.

“We urge them to come clean about it.”

Human rights group Amnesty International adopted the detainees as “prisoners of conscience”.

The government had no immediate comment on the case of the 22 detainees or the ISA.

At the time, the government said the detainees were trying to subvert the social and political system and set up a communist state, and their activities would have led to chaos if unchecked.

The anniversary of the case has again shone a spotlight on the ISA, which allows authorities to detain anyone seen as a threat to security for up to two years.

More than 200 people attended a recent rally to mark the 30th anniversary of the detentions and show of support for a campaign seeking to abolish the law.

Former detainee Tan Tee Seng, 58, who was imprisoned for four months in 1987, said the idea of compensation was not important.

“You can’t undo something that is already done ... I just don’t want my children to live in a place where authorities can detain people without trial,” he said.