The leader of Pakistan's powerful Muttahida Quami Movement (MQM) was arrested Tuesday in London, where he's been living in self-imposed exile since the 1990s.
Reuters says Altaf Hussain was taken into custody in relation to a murder case, but the BBC says he was arrested on suspicion of money laundering.
"Hussain is known for his fiery addresses to his supporters in Karachi though a loudspeaker connected to a telephone. He effectively controls the sometimes violent port city of Karachi from his headquarters in a north London suburb.
"Hussain's hold on Karachi is so strong that he is capable of shutting down entire neighbourhoods of the city of 18 million."
The BBC says:
"Mr Hussain has lived in the UK since 1991, saying his life would be at risk if he returned to Pakistan.
"The British and Pakistani authorities have in the past expressed concerns that any arrest of Altaf Hussain could lead to violent protests in Karachi."
NPR's Philip Reeves, reporting from the capital, Islamabad, says, "News of Hussain's arrest hit Karachi like a thunderbolt. Traffic jams built up as residents, fearful of violence, tried to get home. Shops and gas stations pulled up their shutters; several buses were torched."
The BBC says: "One man in the city, who gave his name as Tahir, told the BBC that MQM supporters were firing guns in the streets and setting fire to any shops which remained open."
On its website, the MQM says its leadership in Pakistan has "advised the party workers to remain calm, peaceful and to stay united."
The MQM is made up primarily of millions of Urdu-speaking Muslims whose families migrated to Karachi when British India was divided into India and Pakistan at the time of independence in 1947.