SINGAPORE - The police are investigating 17 people, including civil society activists, over whether they committed the offence of participating in an illegal assembly.

The group includes sociopolitical website editor Terry Xu and blogger Kirsten Han, said the police in a statement on Saturday (Sept 9).

The incident happened outside Changi Prison Complex on July 13.

The group had held a vigil outside the prison for 29-year-old Malaysian Prabagaran Srivijayan, who was convicted of importing 22.24g of heroin into Singapore and hung at dawn on July 14.

In a Facebook post, Ms Han said police officers at the scene confiscated tea lights and photos of Prabagaran, and the group had complied with their orders.

"We were then told that it would be all right for us to stay, as long as we did not light any more candles. We did not light any more candles," she wrote on Sept 6.

Mr Xu, the chief editor of The Online Citizen website, said he was stopped at immigration on Sept 6 and prevented from leaving the country.

He and Ms Han said they had not been informed earlier that they could not leave the country until after they were questioned by the police.

The move was criticised by the Human Rights Watch in a statement on Sept 7.

"Both the belated criminal investigation and the travel ban have all the hallmarks of a harassment campaign against those who dare to peacefully criticize the government," said deputy Asia director Phil Robertson.

In response to media queries on the travel restriction and typical procedures for such cases, the police said: "Organising or participating in a public assembly without a Police permit is illegal in Singapore. There are rules on where public assemblies can take place, to promote, advocate causes."