And then someone shouted: "It would not have been your right to attack us!" Others then stood up and started chanting: "We want a constitution, now!"
The generals were reduced to pleading with the crowd to remain quiet -- a role they were clearly unused to.
Even as they promised a transition to civilian rule, one general lost his temper and shouted: "When I speak, you listen!"
The event was billed as the first public meeting between members of the Egypt's ruling body, the Supreme Council of the Armed Forces, and the young people of the rebellion.
It was held after the generals found themselves under fire for alleged abuses, with several youth groups announcing a boycott of the meeting in protest at military trials for civilians and because they said the conference lacked substance.
Several dozen held a protest outside the theatre building.
Most of the crowd, however, was clearly sympathetic to the military, offering profuse applause, but a sizeable contingent repeatedly heckled the generals over alleged abuses.
One of the latest accusations came after the American broadcaster CNN reported that a general admitted that soldiers had forced women, arrested during a March 9 protest, to undergo virginity tests. The military has denied the report.
It has also been criticised for summoning journalists over their reporting.
Long confined to barracks, the "top brass" now appears to be struggling as it tries to steer a post-revolutionary country of 80 million people to a parliamentary election in September.
The generals, who appear genuinely earnest, often appear on talk shows and publish their statements on Facebook, one of the networking tools activists used to topple Mubarak.
But such is the growing mistrust among activists that a seemingly well-meaning assurance last week that the military would steer clear of a demonstration was interpreted by some activists as a veiled threat that it would allow thugs to attack the protesters.
At the conference, some officers expressed their shock at seeing generals heckled. "They are shouting at generals," said one lieutenant, in disbelief. "What do they want, a (Libyan leader Moamer) Kadhafi?"
The event ended in a logistical snarl-up that to some appeared a reflection of events in the country.
The military had decided that all cellphones must be deposited at the reception desk. Inevitably, chaos followed as hundreds jostled to retrieve their phones.
The officers eventually persuaded the crowd to return to the auditorium, as a general on the stage yelled out ticket numbers for the phones like an auctioneer.
"This is our future," said one despondent member of the crowd.