“We like it here, everyone is free”: Rafiq Hayat (right), with Philip Breeden, Press Counsellor at the US Embassy, London

As Friday prayers end thousands of believers stream out of the mosque. Keen-eyed men with Pashtun cheekbones hover by a convoy of cars, many of them wearing the traditional hats of Pakistan's North-West Frontier province. Were it not for the Tube depot in the background, one could be in that exotic land with its long and sometimes unhappy links with Britain. Departing last, and driven safely away, is Hadhrat Mizra Masroor Ahmad, the fifth leader of the worldwide Ahmadiyya community.

Built on the site of a former dairy in 2003, the Baitul Futuh in Morden, south London, is the largest mosque in western Europe, with capacity for 10,000 people. When I first visit, around 6,000 believers are packed into the complex, bowed in prayer. There is an airport-style X-ray machine by the entrances, something churchgoers might find alarming, although the Baitul Futuh looks lightly protected compared to the Ahmadiyya mosque in nearby Putney.

There was some local hostility when the mosque opened. Religious centres are rarely welcomed by any community, bringing as they do not just traffic and noise but inevitably more believers to the area, and in a neighbourhood inhabited by many people who fled inner London a generation ago there was bound to be difficulty. Yet the Ahmadiyya are nothing if not good neighbours. The mosque's conference hall is used by various local civic bodies, and every year chocolate is handed out to local people at Eid, a small gesture that nonetheless warms relations. Besides which, the security is not to protect them from BNP supporters.

Ahmadiyya is an Islamic reformist movement that began more than a century ago in British India. Little known outside Pakistan, where half of its ten million followers live, it has nonetheless attracted some attention in Britain for its now annual poppy-selling drive, which raises more than £20,000 a year for the British Legion.

The Ahmadi follow Mirza Ghulam Ahmad, born in the Punjab in 1835, who in 1889 declared himself to be the promised messiah (Imam Mahdi). After his death in 1908 his followers split, with some taking the orthodox Sunni view that there could be no more prophets, and the others, the Ahmadiyya, becoming a new sect. Born on the crossroads of the world's Western and Eastern faiths, they also see Krishna and Buddha as prophets, and this varied religious palate perhaps explains their instinctive tolerance.