Four policemen were killed while providing security to the polio vaccine team in Orangi. Three others were shot dead in another part of the area, police said.
"They were eight assailants on four motorcycles who first attacked a police mobile in which four policemen were on duty," a police spokeswoman said.
"While fleeing after the target killings they also shot at three other policemen," he said.
"The killers apparently were targeting the policemen and not the polio workers who were present at the scene of the first attack during the polio immunisation drive that began today," he said.
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In January, at least 15 people, including 12 policemen and a paramilitary soldier, were killed in a suicide bombing targeting polio workers in Quetta.
Pakistan and neighbouring Afghanistan are the only two countries in the world where polio remains endemic.
Children in Pakistan's Federally-Administered Tribal Areas cannot be vaccinated as the outlawed Taliban has banned the campaigns since June 2012.