London has long been the antechamber of Pakistani politics, where self-exiled leaders take refuge until they can return. The former military ruler Pervez Musharraf lived here until recently, and the current prime minister, Nawaz Sharif, lived here until 2007.

Mr. Hussain, however, shows no sign of going back. The Muttahida Qaumi Movement has an office in Edgware, in northwest London. But these days Mr. Hussain is mostly at home, in a redbrick suburban house protected by raised walls, security cameras and a contingent of former British soldiers he has hired as bodyguards.

From there, he holds court, addressing his faraway followers in a vigorous, sometimes maniacal style, punctuated by jabbing gestures and hectoring outbursts. Occasionally he bursts into song, or tears. Yet, on the other end of the line, it is not unusual to find tens of thousands of people crowded into a Karachi street, listening raptly before an empty stage containing Mr. Hussain’s portrait, as his disembodied voice booms from speakers.

“The cult of personality surrounding Altaf Hussain is quite extraordinary,” said Farzana Shaikh, an academic and the author of “Making Sense of Pakistan.” “He is immensely charismatic, in the way one thinks of the great fascist leaders of the 20th century.”

In Karachi, his overwhelmingly middle-class party is fronted by sharply dressed, well-spoken men — and a good number of women — and it has won a reputation for efficient city administration. But beneath the surface, its mandate is backed by armed gangs involved in racketeering, abduction and the targeted killings of ethnic and political rivals, the police and diplomats say.

Other major Pakistani parties indulge in similar behavior, but the Muttahida Qaumi Movement frequently brings the most muscle to the fight. An American diplomatic cable from 2008 titled “Gangs of Karachi,” which was published by WikiLeaks, cited estimates that the party had an active militia of 10,000 gunmen, with an additional 25,000 in reserve — a larger force, the dispatch notes, than the city police.