On 16 December, all the Cardinals, including Lamberto, assembled in the chapel of the monastery of St. Pancratius attached to the south of the Lateran basilica. There, at the suggestion of Jonathas, the Cardinal-Deacon of Santi Cosma e Damiano, who was a partisan of the Pierleoni family, the Cardinals unanimously elected as Pope the Cardinal-Priest of Sant'Anastasia, Theobaldo Boccapecci, who took the name Celestine II. He had only just put on the red mantle and the "Te Deum" was being sung when an armed party of Frangipani supporters (in a move pre-arranged with Cardinal Aymeric) burst in, attacked the newly enthroned Celestine, who was wounded, and acclaimed Lamberto as Pope. Since Celestine had not been formally consecrated pope, the wounded candidate declared himself willing to resign, but the Pierleoni family and their supporters refused to accept Lamberto, who in the confusion had been proclaimed Pope under the name Honorius II.