At about 15 in the forenoon (11 a.m.) in Santa Maria del Fiore, whilst high mass was being celebrated and the Host elevated, Giuliano, son of Piero, son of Cosimo de' Medici, and Francesco Nori were killed, near the choir of the said church towards the door which goes to the Servi; and Lorenzo de' Medici was wounded in the neck, and fled into the sacristy and escaped. They were killed in consequence of a certain conspiracy made by Messer Jacopo de' Pazzi and Franceschino de' Pazzi and Guglielmo de' Pazzi, the which Guglielma was he brother-in-law of Lorenzo de' Medici, his wife being a sister of theirs, called Bianca. And the sons of Messer Piero de' Pazzi were also there, that is, Andrea and Renato and Niccolo; and of the house of Salviati, there were Francesco, Bishop of Pisa, and Jacopo Salviati, who was son-in-law to Filippo Tornabuoni, and another Jacopo also a Salviati, and Jacopo, son of Messer Poggio, Bracciolini and Bernardo Bandini of the house of Baroncegli, and Amerigo Corsi, and many others. The conspirators brought Cardinal di San Giorgio(1) here, who was a young man; he entered Florence on the day above-mentioned, and they all came together in Santa Maria del Fiore, and, as I have said, at the elevation of the Host seized their swords, and it is said that Francesco de' Pazzi struck Giuliano, and Bandini the other. And having killed Giuliano they wanted to kill Lorenzo, but did not succeed, as he fled into the sacristy. Meantime the Bishop de' Salviati, with Jacopo, son of Messer Poggio, and two of his relatives who were both called Jacopo, went to the Palagio, with several priests, feigning to desire to speak to the Signoria, and they spoke to the Gonfaloniere, and became somewhat confused. The Gonfaloniere perceived the treachery, and he and his companions shut themslves up here and there, and ordered the doors to be closed, and the bell run for a parlamento. And what with the rumour which came from Santa Maria del Fiore of Giuliano's death and the bell ringing at the Palagio, the city was immediately in arms. And Lorenzo de' Medici was taken to his house. Meantime Messer Jacopo de' Pazzi rushed on horseback to the Piazza de' Signori, crying "Popolo e liberta!" (The People and Liberty!), wishing to take the Palagio, but the bishop not having succeeded in getting possession of it, Messer Jacopo was not able to enter. He then went towards his own house, and was advised to take to flight; and he fled by the Porta all Croce, together with many men-at-arms, in the Piazza and at Lorenzo de' Medici's house. And numbers of men on the side of the conspirators were killed in the Piazza; amongst others a priest of the bishop's was killed there, his body being quartered and the head cut off, and then the head was stuck on the top of a lance, and carried about Florence the whole day, and one quarter of his body was carried on a spit all through the city, with the cry of: "Death to the traitors!" That same evening the cardinal was taken to the Palagio, barely escaping with his life, all his companions being captured without exception.
Source: A Florentine Diary, p. 15-17.