At least 40 people were killed, including foreigners and over 70 others injured when an explosives laden truck rammed into a five star hotel here on Saturday.
The wounded, many of them critically, have been brought the local hospitals.
According to sources an Iftar dinner reception hosted by Speaker National Assembly, Fehmida Mirza in the honour of the President Asif Ali Zardari was underway in the hotel when the explosion occurred. Many other important figures were among the guests at the reception.
The powerful explosion caused fire in many parts of the hotel besides shattering the windowpanes of the buildings around the hotel.
Rescue operation is underway to bring out the people trapped inside the hotel.
Emergency has been declared in the hospitals of Rawalpindi and Islamabad.
About 50 to 60 kilograms of explosive material was used in the deadly explosion, sources said.
This hotel in Islamabad is a favorite place for foreigners to stay and gather, and it has previously been targeted by militants.
Ambulances rushed to the scene, where a fire also burned, smoke hovered and the carcasses of vehicles were scattered.
According to senior correspondent of Geo News, Hamid Mir, about 100 people are feared dead in the attack.
There was no immediate claim of responsibility for the blast, and its exact cause was unclear. But Pakistan, a U.S. ally in the war on terror, has faced a wave of militant violence in recent weeks following army-led offensives against insurgents in its border
regions, though the capital has avoided most of the bloodshed.