There was no immediate claim of responsibility, but the explosion happened against the background of a week of street clashes between police and Kurdish protesters in which 16 people have died.
Much of the demonstrators' anger over high unemployment, poverty and Ankara's refusal to grant more autonomy to the mainly Kurdish region has been targeted against the government, and AKP offices have been damaged in the latest protests.
The blast, on Wednesday, came five days after another bomb attack in Turkey's largest city.
That was blamed on Kurdish militants.
Wednesday's bomb blew out the offices' windows in the Esenyurt suburb on the European side of Istanbul, television pictures showed.
Police had no immediate comment.
Fethi Kaya, AKP district chairman, said that "a bomb which was put in front of the door exploded. The building was rendered unusable. Two of our friends were injured".
It was not clear how badly hurt the casualties were.
Bus station blast
Last Friday, a bomb blast at a bus station killed one person and injured 13 others in central Istanbul.
Clashes between protesters and
police have left 16 people dead
The Kurdistan Liberation Hawks (TAK), which has ties to the outlawed Kurdistan Workers Party (PKK), claimed responsibility for the bombing.
A TAK statement this week called on Kurdish youths to "light up the darkness of each evening with fires and other actions. Turkish cities should be covered with black smoke".
There has also been an upsurge in guerrilla violence in the mainly Kurdish southeast.
Security officials said 10 people - six security forces and four PKK members - had been killed in clashes with PKK rebels in recent days.
Five thousand troops backed by Sikorsky and Cobra helicopters launched an anti-rebel operation on Gabbar mountain last week after reconnaissance showed small groups of guerrillas crossing the nearby border from Iraq, a military official said.
The officials said army units were being drafted in from outside to tighten border security in anticipation of an influx of rebels as spring sun melts the snows along the mountainous frontier.
PKK fighters took up arms
against the state in 1984
A 40-strong group of rebels staged an ambush in Sirnak province on Tuesday night, killing four soldiers, including a sergeant.
Two more soldiers died when they stepped on a mine.
During the same operation, four PKK militants were killed in a clash on Cudi mountain where they came under fire from Cobra attack helicopters.
In a separate incident on Tuesday night, rebels attacked a police station with shoulder-fired rockets in southeastern Bingol province, killing one policeman and injuring seven.
More than 30,000 people, mostly Kurds, have been killed in the separatist conflict since the PKK took up arms against the state in 1984 with the aim of carving out an ethnic homeland.
The European Union and the United States, like Ankara, view the PKK as a terrorist group.
In street clashes since last Tuesday, thousands of Kurds have hurled stones and petrol bombs at police who have responded by throwing tear gas canisters and firing bullets above the heads of protesters. Most victims died from gunshot wounds.
Some 720 people have been detained in connection with the street violence, of whom 418 have so far been remanded in custody awaiting trial. More than 300 people have been injured.