The blast occurred on the fourth floor of CitiHotel in Blue Area, one of the main commercial hubs of Islamabad.
Officials of a state-run rescue service said six persons, including a woman, were injured. Two of the injured were in a serious condition, they said.
The injured were taken to the nearby Pakistan Institute of Medical Sciences.
Footage on television showed an injured woman being taken to hospital in an ambulance. Debris and shards of glass lay scattered on the road outside the hotel.
The blast blew out the windows of nearby buildings while the debris damaged several cars in a parking lot. The nature of the blast could not immediately be ascertained.
TV news channels reported the explosion could have been caused by gas cylinders stored in the top floor of the hotel.
Sniffer dogs and a bomb disposal squad scoured the area after the blast, which occurred a few kilometres from the Prime Minister's House, where the country’s top political leadership had gathered for a meet convened by Prime Minister Yousuf Raza Gilani to discuss tensions with the US.
Security across Islamabad had been tightened for the meeting.
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