Thousands of supporters of the Muttahida Qaumi Movement (MQM) have protested in Karachi after UK police arrested the party's leader, Altaf Hussain, on suspicion of money-laundering, Al Jazeera reported.
Hussain, who was detained in London on Tuesday, has lived in self-imposed exile in Britain since 1992 but regularly addresses large gatherings in Pakistan by telephone link.
The MQM is one of the country's main political parties, and has often been accused by opponents of using violence to bolster its power.
The protesters in Karachi, where the party has its biggest base of support, called on the Pakistani government to ensure Hussein's well being.
"We have come here with a demand from our federal government and provincial government that they should talk to the UK government to ensure his safety, health and life," said Farooq Sattar, the MQM parliamentary leader.
After news of his arrest aired on Pakistani television, residents started shutting their businesses and going home, fearful of a backlash, the AP news agency reported.
There were reports of gunfire and protests in the city of Hyderabad, an MQM stronghold 163km north east of Karachi, within minutes of the arrest being reported on Tuesday.
Karachi, a port city of more than 18 million people, is vital to the country's economy, and any repercussions from Hussain's arrest could have serious consequences for Pakistan as a whole.
In 2010, another MQM politician, Imran Farooq, was stabbed to death in a daylight street attack in London, triggered riots in Karachi.
The MQM, formed in 1984, largely represents descendants of Urdu-speaking migrants from India who settled in Pakistan when it was created in 1947.