Police did not identify the suspect but said he was 34 and a Pakistani national who had business ties to Imran Farooq, who had been living in London for more than a decade, The Guardian reported. Investigators were trying to determine the man's political leanings.
Farooq, 50, was a longtime leader in the Muttahida Quami Movement or United National Movement. He applied for asylum in Britain in 1999 with Altaf Hussain, the party's top leader.
Friends said Farooq had been less involved in politics since he became a father. He is survived by his wife, Shumaila Imran, 42, and their sons, who are 3 and 5.
Farooq was a doctor working in a pharmacy. He was attacked as he returned home from work.
Hussain, who lives under tight security, said he believed Farooq's death was the work of "enemies of MQM."
2014: The Year in Music [PHOTOS]