The activists given a warning in lieu of prosecution after they said they were unaware a permit was needed to stage a rally at the city state's designated free-speech corner in Hong Lim Park.

James Gomez, executive director of the reformist group Think Centre, gave reporters copies of a "letter of warning" given by the central police division to him after he was summoned to the station earlier MAr 27.

Gomez had been questioned in connection with a police investigation into a rally on International Human Rights Day on December 10.

A handful of demonstrators, including Gomez, had unfurled banners, raised clenched fists and called for the abolition of the Internal Security Act, which allows for detention without trial.

"This (December 10 rally) is an assembly for which a permit from the police is required. You have stated that you were ignorant that a permit was required for such an event.

"Having considered the circumstances of this case, the police have decided to administer you a stern warning in lieu of prosecution," the letter said.

"You are hereby warned for the said offence and if you commit any similar offence in future, action may be taken to prosecute you in court," it added.

Kevin Liew, program coordinator of the Open Singapore Centre which participated in the rally, said he also received a similar letter.

Gomez told reporters at a briefing at the Speakers Corner that he signed the letter reluctantly after consulting with his lawyer and members of his group.

He said he was told that if he rejected the warning, they could have been prosecuted.

Investigations into the rally had dampened public interest in the Speakers Corner because people were intimidated, he claimed.

He said his group will continue to push for changes on the regulations governing meetings at the Speakers Corner, which opened in September last year but with a slew of limitations, including a ban on the use of microphones.

Police in February warned against the corner being used "as a venue to stage a demonstration or march without a permit, or otherwise to engage in a disorderly behaviour."

"It is one thing to have a group of people gather to hear a person or persons speak, but quite another when people come for a specific cause and in the process they chant slogans, display placards and show gesticulations such as clenching of fists," the police said in February.