Claiming the act allows for detention without trial, a letter to Prime Minister Goh Chok Tong was delivered by opposition leader Chee Soon Juan, seeking an official apology for previous detentions.

Speakers at the gathering mentioned the case of Chia Thye Poh, who they said was detained under the ISA for 23 years for refusing to renounce his communist beliefs.

The small group gathered at Hong Lim Park, Singapore's designated Speakers Corner, to hear speeches demanding the abolition of the ISA and criticising what they said was a clampdown on political freedom in the affluent city-state.

The speeches went largely unnoticed by passers-by and motorists as the use of amplifiers, loudspeakers and music are barred at Speakers Corner.

The activists were forced to settle for the round of speeches after police denied their application for a marathon involving 30-40 runners, fearing it could turn into a procession assembly.

Chee, secretary general of the opposition Singapore Democratic Party, and his sister still ran the full 42 kilometre (26-mile) route without incident, handing the letter to guards at the presidential complex as they passed by.

"It is a matter for regret that the day has in the past gone unobserved in Singapore," said the letter signed by 78-year-old Jeyaretnam, the grand old man of Singapore's political opposition.

The letter appealed to the government "to abolish all detentions without trial whether under the Internal Security Act of the Criminal Law (Temporary Provisions) Act."

"Detention without trial is a violence committed against the people and you should put an end immediately to this violence and express regret about detentions that were committed in the past," the letter said.

Jeyaretnam said he could only confirm two political detainees were still being held under the ISA, which covers any activity "considered prejudicial to the security" of the state.

Speakers at the rally took turns in criticising the absence of political freedoms in Singapore despite its economic prosperity.

Stockbroker Irvin Lim, 30, said the ISA has caused a deep "mental scar" among Singaporeans, most of whom are afraid to ask questions for fear of being reprimanded.

Tan Kong Soon, a 23-year-old student, said the society was more concerned with money and economics, with parents warning their children against being outspoken against the government.