The explosion occurred at the ANP offices in the Bacha Khan Markaz in Peshawar.
Police said that prior to the blast attackers fired a number of gunshots at the ANP headquarters but no one was injured, Dawn newspaper reported.
The attack took occurred amidst of the end of the Durga Puja festival and hundreds of people were present as a result of the festivities, police said. Following the attacks additional police and paramilitary forces were sent from district headquarters.
"Bacha Khan Markaz is the symbol of peace for Pashtuns and is symbolically important for the nationalist party," said ANP Senior Minister Bashir Ahmed Bilour. "The militants want to terrorize people by such attacks but they cannot shatter their will and this headquarters of peace will remain intact."