At least four other policemen were less seriously hurt in the attack in Bani Jamra village, west of the capital Manama, where youths have regularly blocked roads and clashed with police in recent months.
Police say they have been frequently attacked with homemade bombs since April 2012.
Anti-government protesters also blocked a road in nearby Saar area with burning tires.
The Bahraini government has largely crushed the revolt but small-scale protests erupt almost daily.
Tensions have been especially high since security forces raided a top Shi'ite cleric's home on May 17. Hundreds of Shi'ites clashed with police last Friday, and thousands more gathered in the cleric's village for a peaceful sit-in.
The Bahraini revolution began in mid-February 2011, when the people, inspired by the popular revolutions that toppled the dictators of Tunisia and Egypt, started holding massive demonstrations.
Dozens of people have been killed in the crackdown, and the security forces have arrested hundreds, including doctors and nurses accused of treating injured revolutionaries.
A report published by the Bahrain Independent Commission of Inquiry in November 2011 found that the Al Khalifa regime had used excessive force in the crackdown and accused Manama of torturing political activists, politicians, and protesters.