About 18,000 people, mostly young Christians from around the world, attended, reading prayers for victims of war, injustice and sexual violence and calling for defence of the environment.
Hundreds were later starting a three-day retreat north of Rome ahead of the opening of the synod.
Various groups have arrived in Rome to hold news conferences, presentations and protests to illustrate their views, suggestions and demands.
The prayer vigil was attended Christian leaders including Archbishop of Canterbury Justin Welby, spiritual head of the worldwide Anglican communion, and Bartholomew I, the Istanbul-based Ecumenical Patriarch of Orthodox Christianity.
In his homily at that event, he called for "an ever more symphonic and synodal Church".
Persons:
Pope Francis, Francis, Archbishop of Canterbury Justin Welby, Bartholomew I, Philip Pullella, Giles Elgood
Organizations:
CITY, Catholic Church, Peter's, Churches, Thomson
Locations:
St, Rome, Istanbul, Christianity