Majid Kavousifar and his nephew Hossein Kavousifar were executed to shouts of "Allahu Akbar!" (God is Greatest!) when their balaclava-wearing hangmen kicked away the wooden stools on which they stood.
Several thousand onlookers gathered behind a cordon of police and iron railings to witness the execution, held in the exact location of the murder of Hassan Moghaddas in August 2005 in the middle of a busy business district.
Moghaddas, who worked at the "guidance" court which handles sensitive cases of moral corruption, was murdered by the two gunmen when he climbed into his car after work.
Before the execution got under way people stared in fascination at the two blue nooses suspended from five metre long cranes mounted on the back of two pick-up trucks.
They were believed to be the first hangings in Tehran since September 2002.
The hangings brought to at least 151 the number of executions carried out in the Islamic republic so far this year.
Iran's critics accuse the Islamic republic of excessive use of the death penalty but officials argue it is an effective deterrent and is only used after an exhaustive legal procedure.
"Dear beloved people! We are going to start this session. We request you to keep order and praise the prophet Mohammed," an official said through a loudspeaker.
Suddenly, there was a commotion from inside the court building and a group thrust through the crowds of press and police.
It was the black-clad executioners leading the two condemned men to their deaths.
The curiosity amongst the crowd was intense. Several people climbed to precarious positions on top of the iron railings, many taking pictures and video with their cellphones.
The condemned were taken on to the back of the pick-up trucks and the bright blue noose ropes tied around their necks as an official read out a long list of charges in a booming voice through the loudspeakers.
Majid Kavousifar, the mastermind of the murder, showed no remorse, waving one hand from his handcuffs at his co-conspirator at the watching crowds.
He made a show of puffing out his chest, grinning at the crowds and even chatting to the burly black-clad executioner, in an apparent final check of the execution procedure.
The other remained silent and emotionless.
The stools were roughly kicked away - Majid Kavousifar appeared to die almost instantly but Hossein wriggled for some seconds before his body relaxed. "Allahu Akhbar!" yelled the crowds.
"Death to hypocrites! Death to the terrorists! Death to America!" an official boomed from a balcony through loudspeakers.
The two executed men hung limply from the ropes for several minutes, their clothes ruffled by the early morning breeze. They were then taken down and put into a waiting ambulance.