At least 17 more people have been killed as violence continues for a third day in Pakistan's business capital and largest city, Karachi.
In some parts of the city, houses and shops were burnt down in arson attacks. Officials say extra police and paramilitary forces have been deployed.
Pakistani Interior Minister Rehman Malik has ordered police to shoot on sight anyone seen acting suspiciously.
Violence began on Monday after politician Raza Haider was killed.
A Sindh provincial assembly member from the local MQM party, Mr Haider had been performing ablutions in a mosque near the city centre when he was shot dead by four gunmen. His guard was also killed.
The number of those killed in the riots that followed has now risen to 80 and scores more have been wounded.
The city, Pakistan's commercial centre, has virtually ground to a halt since the assassination.
Despite the deployment of security forces, gunmen continue to target civilians and enforce a virtual shutdown throughout the metropolis, says the BBC's Syed Shoaib Hasan in Karachi.
Reports of firing have come in from the Nazimabad, Orangi, Sindh Industrial and Trade Estate and Baldia neighbourhoods.
Tensions had started to recede on Wednesday evening, as some citizens began to venture out for essential fuel and food supplies.
Although some attacks took place overnight, the mood seemed to improve as markets and businesses opened for the first time since Monday's assassination.
But, since early afternoon, gunmen once again forced traders to close.
Banks and other multinational corporations opened their offices but few employees turned up for work. There was little trading at the Karachi Stock Exchange.
"There has been a loss of 20 billion rupees ($235m) in revenue over three days," Haji Abdul Majid, president of the Karachi Chamber of Commerce, told the BBC.
Private schools have announced that they will remain closed until Friday. Government schools were open, but attendance was low.
The University of Karachi, Pakistan's largest educational institution, has postponed regular examinations.
Security officials say they expect the situation to improve after a memorial service for the MQM leader is held on Thursday.
The MQM had announced three days of mourning following the attack.
About 40 people have been arrested in connection with the violence, including about 20 suspected Islamist hardliners who may be implicated in Mr Haider's murder.
Our correspondent says MQM leaders believe that Mr Haider could have been targeted because of the party's strong stance against the Taliban and other militant Islamist groups.