Last year, the government made something of a donation to its own citizens, creating a Speakers' Corner in Hong Lim Park. This move was also intended to provide an outlet, although this time in the political rather than the physical environment. It enabled Singaporeans to exercise their civic muscles a little more easily as they moved into the 21st century.

The experiment was modelled on the forum in London's Hyde Park, where campaigners of every description are free to vent their spleen. Its partial adaptation to Singapore was hailed as a milestone, freeing up the political arena and delivering on Prime Minister Goh Chok Tong's long-standing commitment to promote a more open society. Six months after its creation, however, the authorities are having to make clear the nature of the process they started, reminding citizens of the tight legal framework within which Speakers' Corner operates.

Soon after its September launch, activists attempted to use the new forum as a springboard for more ambitious events, triggering official concern they were over-stepping the mark. Two organisers were called in for questioning, and a police spokesman says inquiries are ongoing.

The issue prompted heated exchanges in parliament and, with a general election on the horizon, may yet resurface. The outcome of the debate will give a clear pointer about the extent of the country's political openness.

James Gomez, head of Think Centre, an independent civil rights group and one of the pair who was questioned, says the corner's creation was not only symbolic but also offered his group a chance to reach a wider audience. He attended its inaugural session, and then helped to promote a series of events at the site, each slightly more complex than the last.

"Very early on we realised that we needed to use visual and verbal techniques," Mr Gomez says. "So we then went on to be more organised. The second time we appeared, apart from a banner, we had colour co-ordinated T-shirts. Then we had either a theme or a number of speakers with a topic to speak on. We went on like this for the next couple of months. Sure enough, every time that we were there, a crowd would be there."

This progression risked running foul of the corner's "rulebook". Under the guidelines, speakers must register with the police ahead of their address, but do not need to state their topic. About 800 people have done so over the past six months, the Ministry of Home Affairs says, adding their addresses are not monitored. Crucially, users of the space do not need to apply for a Public Entertainment Licence to speak outdoors, but they do remain subject to all other laws, including the need to obtain a permit to hold a demonstration. People may come and listen to speakers, it seems, but they cannot gather there with a certain cause in mind.

Walking this legal tightrope was bound to tricky, and feet started to miss the mark on December 10. To mark Human Rights Day Mr Gomez's Think Centre teamed up with the Open Singapore Centre, another civic debating group, to challenge the Internal Security Act, which permits detention without trial. They planned a separate protest marathon that Sunday - which did not get the authorities' permission - but gathered anyway at Speakers' Corner.

"The session started, with me leading three cheers. In the beginning, to get people to come together there was also a photo-shoot, and the speeches went on. All eight [speakers] were registered, at the end we had three more cheers . . .We were all quite happy, we had a good day and thought nothing of it after that. To us it was an activity - it fulfilled the needs of visual and verbal techniques that need to be employed because it's an open-forum environment - and we moved on," Mr Gomez said.

So did the police, although they "moved on" after a six-week gap. Last month, Mr Gomez was called in for questioning, as was Kevin Liew, the Open Singapore Centre's programme co-ordinator. In a strongly worded release, the Singapore Police Force pointed out the rules governing Speakers' Corner were unchanged since its inception, and people were warned not to try turn it into something more inclusive than Mr Goh had envisaged.

"It is not to be used as a venue to stage a demonstration or march without a permit, or otherwise to engage in disorderly behaviour." the police said.

"It is one thing to have a group of people gather to hear a person or persons speak; but quite another when people come together for a specific cause, and in the process, they chant slogans, display placards and show gesticulations, such as clenching of fists. Police treat such actions as indicative of a demonstration or of disorderly behaviour."

The message is clear: Mr Gomez's desire for the space runs some way beyond what the authorities imagined when they moved to set up the forum in Hong Lim Park. His impulse also runs beyond what they are willing to tolerate.

There was no word this week on whether the pair would be charged, or whether police were satisfied that Singaporeans now understand where the boundary between the permissible and illegal is drawn at the park. As Mr Gomez said: "In real terms the ball is still in their court. They can do whatever they want."