Counter-terrorism investigators did not release the name of the man, who was detained after landing at Heathrow International Airport on a flight from Canada, The Guardian reported. Police said he was 52, and some Pakistani newspapers described him as a British national of Pakistani descent.
Farooq was one of the founders of the Muhajir Qaumi Movement, which grew out of a student organization. The liberal political party is especially strong in Pakistan's largest city, Karachi.
After a government crackdown on MQM, Farooq fled to London in 1999. Trained as a doctor in Pakistan, he was working as a pharmacist.
In September 2010, his body was found near his home in North London. Investigators said he had been stabbed with a kitchen knife and beaten with a brick.