Several shots were fired and at least two explosions were heard, as smoke was also seen coming out of the upper floor where Istanbul prosecutor Mehmet Selim Kiraz was being held.
Ambulances were also seen rushing to the entrance of the courthouse but there were no details on casualties. The authorities, who had earlier been negotiating with the captors, did not immediately confirm they had launched an operation against the hostage-takers.
However security forces had been outside the court house building for several hours in possible anticipation of such an operation.
The Turkish government had earlier obtained an order for a media blackout on the standoff and the clashes were not mentioned on Turkish news channels.
The radical leftists had threatened to kill the prosecutor, if their demands surrounding the case of a victim of the police crackdown on 2013 protests were not met.