Gunmen on a motorcycle on Monday killed an elderly Pakistani who had been detained three years ago for suspected al-Qaida links.
Ahmad Javed Khawaja, 72, was attacked after he finished his morning prayers at a mosque and was walking to his clinic in the eastern city of Lahore, said Mohammed Ali, a police official.
The attackers fled and no one has been arrested, Ali said, adding that the shooting's motive was not known.
About 500 people joined a protest that erupted after the shooting. Many chanted, "Death to America!" as they blocked a street and set tires on fire before dispersing after two hours.