Officers admit they are at a loss to know who built or used one of Paris's most intriguing recent discoveries.

"We have no idea whatsoever," a police spokesman said.

"There were two swastikas painted on the ceiling, but also celtic crosses and several stars of David, so we don't think it's extremists. Some sect or secret society, maybe. There are any number of possibilities."

Members of the force's sports squad, responsible - among other tasks - for policing the 170 miles of tunnels, caves, galleries and catacombs that underlie large parts of Paris, stumbled on the complex while on a training exercise beneath the Palais de Chaillot, across the Seine from the Eiffel Tower.

After entering the network through a drain next to the Trocadero, the officers came across a tarpaulin marked: Building site, No access.

Behind that, a tunnel held a desk and a closed-circuit TV camera set to automatically record images of anyone passing. The mechanism also triggered a tape of dogs barking, "clearly designed to frighten people off," the spokesman said.

Further along, the tunnel opened into a vast 400 sq metre cave some 18m underground, "like an underground amphitheatre, with terraces cut into the rock and chairs".

There the police found a full-sized cinema screen, projection equipment, and tapes of a wide variety of films, including 1950s film noir classics and more recent thrillers. None of the films were banned or even offensive, the spokesman said.

A smaller cave next door had been turned into an informal restaurant and bar. "There were bottles of whisky and other spirits behind a bar, tables and chairs, a pressure-cooker for making couscous," the spokesman said.

"The whole thing ran off a professionally installed electricity system and there were at least three phone lines down there."

Three days later, when the police returned accompanied by experts from the French electricity board to see where the power was coming from, the phone and electricity lines had been cut and a note was lying in the middle of the floor: "Do not," it said, "try to find us."

The miles of tunnels and catacombs underlying Paris are essentially former quarries, dating from Roman times, from which much of the stone was dug to build the city.

Today, visitors can take guided tours around a tightly restricted section, Les Catacombes, where the remains of up to six million Parisians were transferred from overcrowded cemeteries in the late 1700s.

But since 1955, for security reasons, it has been an offence to "penetrate into or circulate within" the rest of the network.

There exist, however, several secretive bands of so-called cataphiles, who gain access to the tunnels mainly after dark, through drains and ventilation shafts, and hold what in the popular imagination have become drunken orgies but are, by all accounts, innocent underground picnics.

The recent discovery of three newly enlarged tunnels underneath the capital's high-security La Santé prison was put down to the activities of one such group, and another, iden tifying itself as the Perforating Mexicans, last night told French radio the subterranean cinema was its work.

Patrick Alk, a photographer who has published a book on the urban underground exploration movement and claims to be close to the group, told RTL radio the cavern's discovery was "a shame, but not the end of the world". There were "a dozen more where that one came from," he said.

"You guys have no idea what's down there."