The exiled leader of Pakistan's Muttahida Qaumi Movement, Altaf Hussain, has been arrested in London, sending the country's largest city of Karachi into lockdown.
Scotland Yard said a 60-year old man had been arrested on suspicion of money-laundering during an early morning raid on a house in north-west London, but declined to confirm his identity.
A spokesperson for MQM confirmed that Hussain had been arrested but would not provide further details.
UK and Pakistan authorities have expressed concern that his arrest could unleash a wave of violence in the port city of Karachi, home to 18 million people, which he effectively controls from his London office.
Pakistan officials said protesters in Karachi have blocked roads and destroyed at least six buses and a car following reports of Hussain's arrest this morning. State television showed footage of large crowds gathering on the streets on Karachi and the sound of gunfire.
"All shops and markets are shut. Even small cigarette shops are closed," a Karachi resident told Reuters. "Petrol station operators have also closed down, fearing violence."
Known for his impassioned and fiery speeches, Hussain, a British citizen, has lived in self-imposed exile from Pakistan since 1991.