Rabbi Ramy Banin, a big man with a long grey beard, is sitting in his office switching easily between English, Italian, Hebrew and Yiddish as he chats with visitors who walk through its open door. Outside, the late-summer sun slants across the small square of Venice's Jewish Ghetto, warming the red and yellow walls of the tall buildings and the 500-year-old synagogue, past the memorial to the Jews deported in the Second World War and past shops selling menorah, stars of David and other Hebraica.

A Venezuelan stops by the rabbi's office to introduce his Polish wife - 'a good Jewish girl, of course,' he says. An American woman wanders in and leaves with a handful of pamphlets. Outside, a group of schoolchildren sing a few lines of a religious song.

Banin, 41, is the local 'emissary' of the Chabad or Lubavitch movement, an ultra-orthodox, messianic and controversial strand of Judaism committed to changing the face of Europe's Jewish community. In Britain the long-established Lubavitch community is relatively low-profile. But in central and eastern Europe, and wherever else Jewish life was ravaged by the Nazis, Chabad has launched an evangelical outreach campaign which, helped by huge funds, hundreds of energetic men like Banin and modern marketing skills, is finding big success.

Though barely recognised, the scale of Chabad's operation is vast. When it holds an annual conference at its New York headquarters for its 'emissaries' from around the world, more than 3,000 are expected, each the leader of an established or nascent congregation in America or overseas. Each year dozens of new missions are established.

But as supporters speak of Chabad's activities rejuvenating ageing Jewish communities, detractors are angered by the brash newcomers and their hardline conservatism, theology about the coming of a new messiah and links to the hard right in Israel. In Prague this year, 'Chabadniks' and locals came to blows in one of the main synagogues. There have been tensions in Lithuania, Germany, Russia, Sweden and Italy. When, on a Friday night this summer, Banin entertained up to 300 or so Jews to a free kosher dinner at the Chabad-run restaurant near his office, the guests almost outnumbered the local Jewish community, one of the oldest in Europe. 'They don't like it too much,' said Liav Kohen, the Israeli chef of the restaurant. 'They feel a bit swamped.'

One controversial project has been in Berlin, where Chabad arrived nine years ago and is building a £3m educational centre. Some 20,000-strong a decade ago, the practising Jewish community in Germany has been revitalised by an influx of at least 80,000 largely non-practising Russian Jews. Rabbi Yehuda Teichtal, the Chabad emissary in Berlin, said: 'Many, many people, especially the new immigrants, had no knowledge of their Jewish identity at all and we have been very active in teaching them. Now hundreds, especially children, are coming to our centres and are open and proud about being Jewish,' he said. 'This is our revenge on Hitler. This is the greatest message that the Nazis failed.'

Teichtal denies any friction. Chabad in Germany has many supporters. Nathan Kalmanowicz, a senior member of the German Jewish community's governing body, said Chabad's major donors in the US and Europe provided rabbis for dozens of Germany's 86 small Jewish communities. 'We should do it, but we don't have the resources. The German Jewish community is trying to deal with a massive influx and needs all the help it can get. Chabad complements the existing structures, they don't go against them. They are not my cup of tea, but they don't tell me how to worship and I like them very much,' Kalmanowicz said.

Others take a different line. '[Chabad] is a danger because it is like a sect,' said Dr Julius Schoeps of Berlin's Moses Mendelssohn Centre for Jewish Studies. 'They have the money, they have a message and a very effective educational system. It is a sort of war. They conquer the countries where Jews are living ... and that has profound, long-term consequences for Jewish communities throughout Europe.'

Schoeps, like many other Jews, is concerned about the messianic message. Chabad started in the Belarussian town of Lubavitch 250 years ago, led by rabbis who told their followers that they should prepare for a messiah who would deliver the Jews and usher in a new age of global peace. After decades on the margins, Chabad, under the leadership of a brilliant scholar called Menachem Mendel Schneerson, began to grow rapidly. Images of Schneerson appear in the movement's literature. A picture hangs in Banim's office and his restaurant.

Most controversially of all, Schneerson, who fled Nazi persecution to the US, where he died in 1994, is considered by many to be the messiah. Some believe he lives on, though not in a physical form. 'Messianism is a huge component of their philosophy - as is outreach, all mixed with an energetic, lively, happy idea of what it means to be Jewish that touches a whole range of ethnic and religious identity issues' said Dr Wilson Pickett, of the Institute for Jewish Policy Research London. 'Put it together and you have something that is far from bland, spiritual fare and can be very attractive.'

But though it is camouflaged by mysticism, and emphasises 'a constructed chicken soup identity', Chabad has a political side too. It staunchly supports Israel's Jewish settlements in Gaza and the West Bank, and members were at the forefront of protests during the forced withdrawal of settlers last month. Chabad leaflets in the Venice office talk of 'the integrity of the land of Israel ... given by the Almighty to His people in perpetuity.'

But emissaries mainly concentrate on raising consciousness, as among Jewish tourists in Venice, and on raising a new generation. In Venice, Banim proudly says, Chabad has just opened a kindergarten. There are just a handful of children but the rabbi hopes numbers will rise. Other activities include tha parading of a 25ft candle-holder or menorah, placed on a gondola and taken round the canals to celebrate the Jewish holiday.

Banim admits there have been difficulties - 'people are just scared of something new' - but denies planning to 'change anything'. 'We want to bring back what was once here. We came here and found a place that was history, it is not by closing yourself in that you can grow. Things are getting better and better, and we will eventually blend into one Jewish community in Venice.'

All observers - and 'chabadniks' - agree that both the mission and the controversial nature of the movement are rooted in the experience of European Jews in the 20th century and the spiritual environment for all religions in the increasingly secular modernity of the 21st century.

'For a long time Jews just concentrated on survival,' said Banim. 'Now everything is available, and the challenge is being Jewish though you could be something else.'