The attack took place on the first day of a four-day local anti-polio campaign in Orangi. Unidentified gunmen riding a motorbike opened fire at the team and then fled, police said.
"Two gunmen opened fire at the policeman who was on duty to guard the vaccinators," Ali Asif, a senior police official in the area, told AFP, adding that the vaccination campaign in the area had now been postponed.
The policeman is critically ill after a bullet passed through his skull, Doctor Mohammad Abid in the city's Abbasi Shaheed hospital told AFP.
Pakistan is one of only three countries where polio remains endemic. Attempts to stamp it out have been badly hit by opposition from militants and attacks on immunisation teams that have claimed 68 lives since December 2012.

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