The policemen were returning to a police station after a vaccination campaign on the outskirts of Peshawar when they were targeted, officials said.
No group claimed responsibility for the incident but similar attacks in the past have been blamed on the Pakistani Taliban and other militant groups that are opposed to the anti-polio campaign.
Last week, seven teachers were kidnapped by the banned Lashkar-e-Islam during an anti-polio campaign in the Khyber tribal region. They were released later.
On June 19 last year, the Taliban imposed a ban on polio vaccinations in parts of the tribal belt and said the restriction would last till US drone attacks cease. At least 260,000 children in North and South Waziristan have not been vaccinated since July 2012.
Pakistan is one of only three countries where polio is endemic. The number of cases in the country reached 62 this year, up four from the whole of last year.