The party condemned the arrest of Dr Asim Hussain – a petroleum minister in the past PPP-led government and close friend of former President Asif Ali Zardari - as politically motivated.
A senior security official in Karachi confirmed that Hussain was being held but would not say under what charges. He spoke on condition of anonymity because he was not authorised to release the information.
Most of the hundreds of people targeted by paramilitary forces in recent months on accusations of political corruption and violence have belonged to the Muttahida Qaumi Movement (MQM), which dominates Karachi politics but plays a smaller role on the national stage.
Read: Dual nationality: Dr Asim Hussain resigns from Senate
The first high-profile arrest of a PPP figure raised new accusations that Pakistan's powerful military is seeking to strengthen its grip on the country's largest and wealthiest city and weaken civilian parties. "These policies will lead to chaos," said PPP lawmaker Khursheed Shah.
The paramilitary Sindh Rangers have in the past said the Karachi crackdown is necessary to break the cycle of violence and corruption in the teeming metropolis of 20 million people.
Karachi accounts for half of Pakistan's national revenues and hosts the stock exchange, central bank and two ports. It is also a hub of violence, often linked to political rivalry.
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