They entered through Russell Square station. For 10 minutes, the four of them sprinted along the tracks of the Piccadilly line towards a disused tunnel at Holborn. Their prize: a sight of one of the great trophies of London's urban exploration scene – the abandoned platforms of Aldwych tube station.

The expedition last year was supposed to be the second last stop in a tour of the capital's 18 "ghost" tube stations. Instead it has sparked a legal battle over the human rights of a community of photographers dedicated to visually documenting restricted areas across the world – and pointing out security loopholes.

To avoid a regular tube service, the explorers chose Easter Monday – four days before the wedding of Prince William and Kate Middleton. Above ground, one of the biggest security operations in the history of the Metropolitan police was swinging into action at an estimated cost of £20m. Officers searched lampposts and traffic lights for hidden bombs; 35 sniffer dogs scoured for traces of explosives; armed commandos trained to counter gun attacks and squads of police monitored the internet for potential plots.

And below the city, four members of the London Consolidation Crew exploration collective were running on the tracks. Their expedition was between 2am and 3am, when only maintenance trains are in use. But as one explorer, Otter, wrote on his website, Silent UK: "At any moment the track on which we stood could have gone live, its guest of honour a 40mph mass of iron and steel singing our last goodbyes."

From Holborn they noticed the rails turn rusty and saw piles of flyers collecting at the tunnel's edges. And then, like hikers who'd reached the best view from the mountain, they saw the forest-green tiles of the platform edge.

For the next four hours they photographed the ticket halls, deserted walkways and antique lift system. Like their other trips – to the roof of St Paul's cathedral, the London Olympic Stadium, Battersea power station – they were careful to leave things as they found them; graffiti is taboo for urban explorers. When the battery on their camera went flat, they got ready to leave. They were interrupted by a shout: "Get on the ground!"

CCTV operators had alerted British transport police, who had issued a terror alert. After infiltrating 200 sites across the city over 10 years and getting away with it, they were busted.

"Normally we would have been dished off to the graffiti squad," Otter says. "But because of the wedding we ended up with detectives much higher up."

The explorers were put in cells and interviewed. Their laptops, cameras and hard drives were confiscated. Otter says: "The police pretty quickly realised our intentions and let us go with a caution."

Three months later an unassociated group of explorers was arrested after accidently derailing a small electric train on a one-off joyride on London's mail rail, a 23-mile underground network that carried post until 2003. The incident sparked an ongoing court case alleging damage to government property and aggravating vehicle taking.

Trespassing

According to Bradley L Garrett, an urban explorer who is writing a PhD on the phenomenon, the mail rail incident reignited Transport for London's interest in the Aldwych four.

Last month TfL applied to issue anti-social behaviour orders which would not only stop them undertaking further expeditions and blogging about urban exploration but also prohibit them from carrying equipment that could be used for exploring after dark. Extraordinarily, it also stipulates they should not be allowed to speak to each other for the duration of the order – 10 years.

"To me, telling people they can't associate with their closest friends is an incredible invasion of human rights," says Garrett. "It's a complete overreaction and an amazing tack to take after the group already agreed to a caution." He thinks TfL's legal action is fuelled by a wider misunderstanding of what urban exploration is about. "What we do is very benign," he says. "The motivation for it comes from a love for the city – we want to interact with its hidden histories and forgotten stories and places."

A TfL spokesman said: "Trespassing on the tube network is illegal and extremely dangerous not just for the safety of the trespasser but also for the security of the railway. As several elements of the legal proceedings regarding the individuals who trespassed at Aldwych are ongoing we will not able to comment further until those have concluded."

Garrett disagrees that the asbos will protect the explorers from themselves. "Urban explorers operate with almost the same safety that track workers operate with," he says. "We have the equipment, we know what we're doing, we have the map of the entire system memorised – we don't take unnecessary risks."

The issue of under-prepared copycats is a sore point among urban explorers. In 2005, a 19-year-old woman died after breaking away from an underground party in the Odessa catacombs, a 2,500 mile tunnel network in Ukraine. Experienced explorers like those at Aldwych undertake painstaking research, training and equipment checks before each trip.

For Garrett, part of the goal is helping to iron out the security loopholes they exploit. But this "service to the city" has proved a double-edged sword. "What this all comes down to is the Olympics because what we're doing could make London's security seem weak, which is embarrassing for TfL," he says.

"But rather than stifling our free speech to tell Londoners there are security weaknesses all over the system, they should probably call us and bring us on as consultants to help fill these gaps."

Despite the legal battles and tightening security, London's urban explorers do not seem deterred.

"We're as active as we've ever been, we just don't share anything any more. We've been driven underground," Garrett says without acknowledging any irony. "Feeling you are pushing your mental and physical boundaries is what keeps people healthy and interested and turns them from inhabitants of a city to citizens – we are active citizens of London."