But there is more than money laundering that Husain is being accused of, say observers.
A BBC documentary aired on Thursday also suggested that the MQM leader may have had a hand in the murder of one of his close associates, Dr Imran Farooq, in 2010.
But the MQM denies any involvement.
Husain, a British national, told his supporters in Karachi over the phone that he would cooperate with the British authorities but at the same termed the move as a conspiracy against him and his party.
Fearing protests and violence in Karachi against the arrest, the Pakistan government said it had taken preventive measures. Officials said they had received reports of targeted killings in Karachi by MQM supporters.