The veneration of icons had been banned by Byzantine Emperor Constantine V and supported by his Council of Hieria (754 AD), which had described itself as the seventh ecumenical council. The Council of Hieria was overturned by the Second Council of Nicaea only 33 years later, and has also been rejected by Catholic and Orthodox churches, since none of the five major patriarchs were represented. The emperor's vigorous enforcement of the ban included persecution of those who venerated icons and of monks in general. There were also political overtones to the persecution—images of emperors were still allowed by Constantine, which some opponents saw as an attempt to give wider authority to imperial power than to the saints and bishops. Constantine's iconoclastic tendencies were shared by Constantine's son, Leo IV. After the latter's early death, his widow, Irene of Athens, as regent for her son, began its restoration for personal inclination and political considerations.
In 784 the imperial secretary Patriarch Tarasius was appointed successor to the Patriarch Paul IV—he accepted on the condition that intercommunion with the other churches should be reestablished; that is, that the images should be restored. However, a council, claiming to be ecumenical, had abolished the veneration of icons, so psychologically another ecumenical council was necessary for its restoration.
Pope Adrian I was invited to participate, and gladly accepted, sending an archbishop and an abbot as his legates.
In 786, the council met in the Church of the Holy Apostles in Constantinople. However, soldiers in collusion with the opposition entered the church, and broke up the assembly. As a result, the government resorted to a stratagem. Under the pretext of a campaign, the iconoclastic bodyguard was sent away from the capital – disarmed and disbanded.
The council was again summoned to meet, this time in Nicaea, since Constantinople was still distrusted. The council assembled on September 24, 787 at the church of Hagia Sophia. It numbered about 350 members; 308 bishops or their representatives signed. Tarasius presided, and seven sessions were held in Nicaea.