At least 20 people have been killed and dozens injured in an explosion at a mosque during Friday prayers in Pakistan's capital, Islamabad, according to local officials.

The blast at the Shiite mosque in the Tarlai area of the city prompted an emergency to be declared at three hospitals in the city, with officials giving figures for the injured ranging between 80 and more than 100.

The cause of the blast was not immediately clear; however, reports have suggested it was carried out by a suicide bomber.

Pakistan's Prime Minister Shehbaz Sharif has condemned the incident, expressing deep grief.

This is a developing story and will be updated soon.