The leader of Pakistan's MQM party, Altaf Hussain, has been arrested in London on suspicion of money-laundering, reports said Tuesday as panic spread through his home city of Karachi and protesters torched vehicles.
British police said only that a 60-year-old man had been detained at a residential address in northwest London and officers were searching the building.
"Officers have this morning arrested a 60-year-old man on suspicion of money-laundering," a Metropolitan Police statement said.
Hussain fled Pakistan for Britain in 1992 after a military operation to end ethnic unrest in Karachi, gaining British citizenship in 2002.
The Muttahida Qaumi Movement (MQM) has its power base in the violence-racked city of Karachi, Pakistan's largest, which it controls with an iron fist.
Panic gripped the sprawling metropolis of 18 million people soon after local television broadcast the reports. Angry protesters torched at least a dozen vehicles, according to rescue officials.
Many rushed to stock up on groceries in anticipation of a prolonged shutdown while office workers left for home early, clogging up roads.
"We don't know for how long the shops will remain closed and I want to store as much groceries as I could," Razia Begum, 45, said as she jostled for space in a packed shop.
A spokesman for Pakistan Railways told AFP that all trains leaving Karachi had been temporarily halted.
The MQM's parliamentary leader inside Pakistan meanwhile appealed to party workers to remain calm and pray for Hussain.
"We should stay calm, we should not become impatient in any condition. The workers must be aware with the developments, keep in touch with the party office, stay united and also look around you," said Dr Farooq Sattar.
"We should act on the teachings of Altaf Hussain. We should pray for his health, we should say that he should be given medical treatment and his medical tests should be conducted," he added.
British officials said they had temporarily closed their deputy high commission in the city, as Karachi police strengthened security in the city's diplomatic enclave.
"We have tightened the security of the consulates, especially of the British," said city police chief Ghulam Qadir Thebo.
The Karachi Stock Exchange plunged three percent on the news, from 29,784 points to 29,009 points.
Karachi is Pakistan's economic heart but is frequently rocked by ethnic, sectarian and militant violence and has one of the world's highest murder rates.
The MQM party was created to safeguard the rights of the city's Urdu-speaking community, who migrated from India to the newly created Pakistan at the time of partition in 1947.
Hussain's residence in London was raided on suspicion of money laundering in 2012 and 2013 by British police.
In 2010 the murder in London of Imran Farooq, one of the party's founding members and a confidante of Hussain, sparked speculation of an "inside job".