Adrian accidentally discovers this conspiracy, and indignantly remonstrates with the barons, threatening even to denounce them, since they are about to break their word and resort to such dishonourable means.  But his own father, Colonna, is one of the instigators of the conspiracy, and he dares him to carry out his threat, which would only result in branding him as a parricide.  Then, without waiting to hear his son’s decision, the old baron, accompanied by the other conspirators, joins Rienzi on the balcony, whence he has just addressed the assembled people.  They have been listening to his last proposal, that the Romans should shake off the galling yoke of the German Empire and make their city a republic once more, and now loud and enthusiastic acclamations rend the air.

The speech ended, Adrian, stealing softly behind the Tribune, bids him be on the watch as treachery is lurking near.  He has scarcely ended his warning and slipped away ere the conspirators suddenly surround the Tribune, and there, in the presence of the assembled people, they simultaneously draw their daggers, and strike him repeatedly.  This dastardly attempt at murder utterly fails, however, as the Tribune wears a corselet of mail beneath the robes of state, and his guards quickly disarm and secure the conspirators while the people loudly clamour for their execution by the axe, a burly blacksmith, Cecco, acting as their principal spokesman.

Rienzi, who is principally incensed by their attack upon Roman liberties, and by their utter lack of faith, is about to yield to their demand, when Irene and Adrian suddenly fall at his feet, imploring the pardon of the condemned, and entreating him to show mercy rather than justice.  Once more Rienzi addresses the people, but it requires all his persuasive eloquence to induce them, at last, to forgive the barons’ attempt.  Then the culprits are summoned into the Tribune’s august presence, where, instead of being executed as they fully expect, they are pardoned and set free, after they have once more solemnly pledged themselves to respect the new government and its chosen representatives.  This promise is wrung from them by the force of circumstances; they have no intention of keeping it, and they are no sooner released than they utter dark threats of revenge, which fill the people’s hearts with ominous fear, and make them regret the clemency they have just shown.

The next act is played on one of the public squares of Rome, where the people are tumultuously assembled to discuss the secret flight of the barons.  They have fled from the city during the night, and, in spite of their recently renewed oaths, are even now preparing to re-enter the city with fire and sword, and to resume their former supremacy.  In frantic terror, the people call upon Rienzi to deliver them, declaring that, had he only been firm and executed the nobles, Rome would now have no need to fear their wrath.  Adrian, coming upon the spot as they march