Pakistan Soldiers Raid MQM Office
Security forces have raided the Karachi headquarters of Pakistan's fourth biggest political party, sparking street protests.
Col Tahir Mahmood said troops acted on information that the Muttahida Qaumi Movement (MQM) was hiding criminals.
A number of party workers suspected of murder were detained, he said, and illegal weapons were recovered.
The MQM has long been accused of using violence and intimidation to control Karachi - claims it denies.
The party's leader Altaf Hussain, who lives in self-imposed exile in London, is being investigated in the UK over money laundering allegations.
His arrest by UK police last year sparked protests in Karachi.
After Wednesday's early morning raid, supporters and activists gathered outside the MQM headquarters and party leaders called for a nationwide strike.
Speaking to Pakistan's Geo News, senior MQM figure Farooq Sattar said the operation was "inappropriate and beyond understanding".
"MQM has itself always maintained its policy of zero tolerance against crime and terrorism, and it has always offered to co-operate," he said.
The MQM says a number of its leaders, including senior leader Amir Khan, have been detained.
But Col Tahir told local media Mr Khan had not been arrested, but was being held for questioning to explain "the presence of criminal elements".
Col Tahir said a large cache of weapons had been recovered, including illegal weapons that "cannot even be imported into Pakistan".
"We suspect that these weapons may be those from missing Nato containers," he said.
Party officials said the weapons were held legally.