(CNN) -- The London-based leader of one of Pakistan's most powerful regional parties was arrested at his home Tuesday in connection with a money laundering probe, officials of his party said.
Altaf Hussain, leader of Pakistan's Mutahida Qaumi Movement, was arrested by London's Metropolitan Police, MQM Senior Deputy Gov. Nadeem Nusrat told reporters in Pakistan.
Hussain, 60, leads MQM from the United Kingdom, where he sought political asylum in 1991. MQM is the fourth-largest party in the Pakistani Parliament, holding 25 out of 446 seats, though it is a greater force in Sindh province, including Pakistan's largest city, Karachi.
Metropolitan Police said they arrested a 60-year-old man at a home in northwest London on suspicion of money laundering Tuesday morning, but they did not release his name.
Hussain has been ill and was preparing to leave his home to get checked at a hospital when he was arrested, Nusrat said.
"Workers and supporters all over the world, including Pakistan, are very concerned and worried for his welfare," Nusrat told reporters.
Metropolitan Police said the man they arrested "has an existing medical condition," and that they were taking him to a hospital for a previously scheduled medical appointment.
"He will remain in custody of officers throughout," the police said in a written statement.
Details of the money laundering allegations weren't immediately available. Faisal Subzwari, a senior MQM politician, told CNN the party is cooperating with the Metropolitan Police investigation.
After news of the arrest, MQM called for protests in Karachi, which turned violent. The party condemned the violence, blaming "miscreants," who have nothing to do with MQM, for what happened.
The Prime Minister's office similarly condemned the violence.
"Though, people are restive over the arrest, Law does not warrant the loss of people's lives and properties on the pretext of reaction to the incident. The Prime Minister has directed the Government of Sindh and the law enforcement agencies to ensure the safety of lives and properties of people in Karachi," it said in a statement.
MQM is based in Karachi and is a force in Sindh province's politics. It has the second-largest contingent in the provincial assembly, with 37 out of 130 seats. It styles itself as a voice for the downtrodden, the middle class and immigrants from nearby India, but political rivals accuse the party of using heavy-handed tactics, including intimidation, to stay in power.
Hussain sought political asylum in the United Kingdom in 1991 because of an attempt on his life, the party has previously said.