Feroze Shah, deputy Inspector General Police, said the policemen were escorting teams giving polio vaccines to children in Orangi Town area when they came under attack, Xinhua news agency reported.
He said the attackers first gunned down three policemen who were on foot and then opened indiscriminate fire at a police mobile, killing four more policemen and injuring three others.
No polio worker was killed or injured in the drive-by shooting by four militants on two motorcycles.
The attackers fled the scene. The police have launched a search operation in the area.
The injured were rushed to a hospital where the medico-legal officer said they were in critical condition.
The three-day polio eradication drive, which started on Monday, was suspended following the incident.
No group has claimed responsibility for the attack yet.
Prime Minister Nawaz Sharif condemned the killings and directed hospital authorities to provide best possible medical treatment to the injured.
Pakistan and Afghanistan are the only two countries in the world where the polio virus is yet to be eradicated.
The anti-polio campaign in Pakistan has been badly affected by militants who attack polio teams and threaten parents against allowing health workers to administer polio vaccine to their children.
An estimated 80 workers have been killed by militants on polio teams since December 2012.