Pakistani paramilitary forces raided the headquarters of a powerful political party in the port city of Karachi on Tuesday after supporters of the party stormed the office of a television channel the day before.
The operation deepens a dispute between Muttahida Qaumi Movement (MQM), which has dominated Karachi politics for decades, and local security forces that regularly spills into violence.
It also comes as Karachi, a metropolis of 20mn people and home to the stock exchange and central bank, is set to formally swear in imprisoned MQM politician, Waseem Akhtar, as mayor on Wednesday.
Senior MQM leader Farooq Sattar, who was detained on Monday after the attack on the TV channel, sought to distance himself from comments made by the party's influential, firebrand leader Altaf Hussain, who lives in exile in London.
Sattar said that Monday's violence, during which MQM supporters fired shots at the office of a television channel, had been started by Hussain when he criticised Pakistani media in a telephone address to his supporters in Karachi.
The supporters later clashed with police outside the television building, leaving one person dead and several wounded.
"Whatever happened yesterday should not have happened, we condemn it," Sattar told reporters after his release. "MQM will be run from here (Pakistan). This message is for there (London) and it is for here."
Paramilitary Rangers forces sealed the MQM headquarters and media office on Tuesday.
In a statement issued from London on Tuesday, Hussain asked for forgiveness from the army and Rangers chiefs.
"From the depth of (my) heart, I beg pardon from the Pakistani establishment," he said. "I was under severe mental stress over extra-judicial arrests and (the) precarious condition of my workers sitting at (a) hunger striking camp."
Hussain is known for fiery addresses to his followers in Karachi though a loudspeaker connected to a telephone in his London home.
Many residents have grown increasingly intolerant of MQM's street protests, which can severely disrupt life for millions of Karachi residents.
But the party remains a major electoral force and in December swept local elections, including jailed leader Akhtar.
The MQM largely draws its support from the descendants of Urdu-speaking migrants from India who dominate Karachi and other urban centres of southern Sindh province.
The army has long accused the MQM of racketeering, kidnappings for ransom and targeted killings in Karachi.
MQM denies the charges and has accused the Rangers of carrying out extra-judicial killings of its members since a crackdown on crime in Karachi began in 2013. The Rangers deny any abuse.