The incident took place in Golra on the outskirts of Islamabad Saturday, Dawn News reported.
Police said the man hired the services of a sex worker and brought her to a friend's house.
However, he was spotted by the people of the area who assembled outside the house and broke into it.
On finding the three people in an objectionable condition, more then 20 people beat the three using iron rods and batons.
However, they later took all the three injured to a hospital, where one of the injured died. The condition of the sex worker was said to be critical.
Police have arrested some of the assailants.