About 40,000 young people from many European countries are expected to join in the 29th gathering, called "a pilgrimage of trust on earth".
So far, about 28,000 pilgrims from Poland, Germany, Italy, Romania, France, Ukraine, Portugal, Lithuania, Hungary, Spain and Bosnia-Herzegovina have flocked in Zagreb.
They will be accommodated in the homes of parishioners of 165 parishes in the area of Zagreb and its surroundings.
It is common practice that pilgrims - Catholics, Protestants, and Orthodox believers - take part in prayers and religious services.
This year the Islamic faithful In Zagreb will attend for the first time masses and prayers that are scheduled within the 29th meeting of the youth.
On 31 December, the primate of the Catholic Church in Croatia, Cardinal Josip Bozanic, and the brethren from Taize will say Mass in the Zagreb Cathedral, and the organisers of the pilgrimage have said the religious services will be covered live by Eurovision.
From its beginning the Taize community has been inspired by two aims: to live in communion with God through prayer and to be a leaven of peace and trust in the midst of the human family.
The Taize community was established by Brother Roger in 1940.