Police in London have arrested the leader of Pakistan's MQM party, Altaf Hussain, on money-laundering charges, the BBC understands.
Officers are searching a residential address in north-west London, where a 60-year-old man was detained.
Mr. Hussain has lived in the UK since 1991, saying his life would be at risk if he returned to Pakistan.
Police in Karachi, Pakistan's largest city, which the MQM controls, have been put on alert amid reports of violence.
Shootings have been reported from some parts of the city, which BBC correspondents say is tense.
Traffic jams are reported in Karachi and other cities in Sindh province as businesses start to close and people head home fearing violence.
Security is being tightened around the British mission in the city and other buildings.
Muttahida Qaumi Movement (MQM) party leaders are holding an emergency meeting at their headquarters in Karachi.
The city has been wracked by violence - much of it politically motivated.