Where the faithful and non-believers flock to Masses with a brotherly touch of opera

It is Sunday morning and the Eglise Saint-Gervais is filled to overflowing. Parishioners sit in the aisles and on the cold stone steps of side chapels, or stand at the back. Many carry their own little prayer benches, and join the flock of nuns in white headscarves, blue habits, cream-coloured capes and sandals who kneel on grass mats before the altar.

The entire Mass is sung - not spoken - by monks and nuns with crystalline voices. They are accompanied by the oldest organ in Paris, made famous by eight generations of the Couperin family between the 17th and 19th centuries.

To my right, an elegant Parisian in an expensive fur-lined coat closes her eyes with a pained look of concentration. A few rows ahead of her, an African in a frayed trench coat who looks like a homeless man smiles beatifically and raises his hands to the incense swirling in a beam of sunlight.

Around me there are worshippers from every social class, race and age group. When her husband was mayor of Paris, it is said that France's first lady, Bernadette Chirac, used to slip across the street from the Hotel de Ville for Mass. In a country where most people go to church only three times in their lives - to be baptised, married and buried - the success of the Eglise Saint-Gervais would give any bishop cause for envy.

Cardinal Francois Marty entrusted Saint-Gervais to Brother Pierre-Marie Delfieux, the founder of the Monastic Communities of Jerusalem, in 1975. (As a sign of humility and a rejection of rank, all of the priests and nuns call one another brother and sister - not father or mother.) Brother Pierre-Marie had been the chaplain of the Sorbonne for five years, then lived for two years as a hermit in the Sahara desert. "But I was too happy," he told French television. "And I realised that was not the real desert. You find real solitude in the heart of big cities."

To establish a monastic presence within the city, Brother Pierre-Marie's followers live in rented apartments and hold part-time jobs outside their community. From 25 different countries, they tend to be young and well educated, and work as teachers, nurses, administrators and even civil servants.

"It's out of a spirit of solidarity," explains Brother Patrick O'Mahony, whose family emigrated from Cork to France in the 18th century. "We want to be city-dwellers among the city-dwellers. We want to be in the world without being of the world." The 39-year-old priest, who celebrated Mass at Saint-Gervais on Sunday, studied political science and was an actor before his St Paul-like conversion in 1985.

The Jerusalem Communities were kept under observation by the Catholic Church until 1991, when their unique liturgy was formally authorised by the Archbishop of Paris. Today, the order's 130 priests and nuns live in five French towns. Last year, they established a house in Florence, and will move to Brussels by the end of this year. They have long-term ambitions of going to Istanbul - "a first bridge to Jerusalem, the cradle of the Byzantine tradition", Brother Patrick says.

At the end of Mass, parishioners raise their arms wide towards the heavens. Then they turn to one another and embrace, saying "the peace of Christ", most with radiant faces. Monks and nuns move through the church, clasping each visitor by both hands and talking to them. This is touchy-feely Christianity, Catholicism at its most fraternal. "We want to include the body," Brother Patrick explains later. "So we raise our hands higher, prostrate ourselves on the ground. Prayer is not an exclusively mental activity. To involve the senses we have icons for the eyes, incense for the nose, music for the ears. It is very physical."

The order has made its liturgy as ecumenical as possible. The "people of God" chants it uses, written by Father Andre Gouze, draw on Gregorian and Byzantine chants, Lutheran chorales, even Basque, Breton, Irish and Polish folk songs. From Judaism, Brother Pierre-Marie took the large, 1.7 metre menorah that stands behind the altar of Saint-Gervais.

To give the nuns a role in the liturgy, he followed the Jewish tradition of having women light candles. They also hold the chalices for Communion. In France, incense is used only in cathedrals and at funerals, but the Jerusalem Communion has adopted the Eastern Orthodox custom of constantly permeating the sanctuary with its fragrance.

Non-believers also enjoy the operatic, theatrical quality of the Mass. Xavier Ternisien, religion correspondent for Le Monde, says the singular beauty of the liturgy is what most attracts people to Saint-Gervais. The order "emphasises spirituality more than social work, and are very focused on prayer. Yet there's something very modern about it; you can drop in but you don't have to stay. It fulfils a need for beauty and spirituality, but leaves you free. You don't feel trapped."