Acting Inspector General of Police in Islamabad Khalid Khattak said there was no prior information about the attack from intelligence sources.
Police launched a search operation and cordoned off the area. The attack is suspected to be an act of terrorism.
On suspicion, police have arrested a man from the location of the blast, according to Dawn.
The attack came a day before the Tehrik-e-Taliban Pakistan’s (TTP) ceasefire ends April 10.
Intelligence agencies had first alerted police and government officials April 3 about possible terrorist attacks in Islamabad, Peshawar and Quetta by militants opposed to the peace talks between the government and the TTP.
“Law enforcing agencies will be the main target of the terrorists, and in case of their failure, any other official installations or public places,” Dawn quoted one police official as saying.
“TTP will choose the targets and the informal groups under its umbrella will provide support to the Ansarul Hind activists with weapons and other materials,” he added.
The newspaper, citing security sources, said the scenario looked similar to the terrorist attacks that quickly followed the announcement of a month-long ceasefire by the TTP last month.
The most audacious of them was the bomb-and-gun attack on the district courts complex in Islamabad March 3, for which the Ansarul Hind group claimed responsibility.