Diplomats said it appeared unlikely there would be another vote to try to renew the mandate before it expires on April 30
The intruders smashed windows on the top floor of the building in the Kartal district on Istanbul's Asian side and hung out a Turkish flag with a white sword motif added to it, footage on the private agency's website showed.
One of the two, a man, occasionally shouted from the window to a crowd below but his words were inaudible.
Police cars gathered outside the building.
The incident occurred as judges and lawyers congregated in a courthouse on the European side of the city to mourn the prosecutor who was killed during an attempt by police special forces to rescue him on Tuesday.
The two hostage-takers - members of the banned far-leftist Revolutionary People's Liberation Party-Front (DHKP-C) - were also killed in the rescue effort.