The Catholic Church has called for a global day of fasting and prayer on Saturday for peace in Syria and against any armed intervention, with Pope Francis scheduled to host a mass vigil on St Peter's Square.
The Argentine pope has called for a "cry for peace" from the world's 1.2 billion Catholics, Christians as a whole, believers from other faiths and atheists.
Syria's Sunni Muslim leader, Grand Mufti Ahmad Badreddin Hassoun, has called for Syrians to join in the prayers and the patriarch of Constantinople Bartholomew I, spiritual leader of the world's Orthodox, has said he is also backing the initiative.
A giant peace flag will also be raised in Assisi, the hometown of the patron saint of peace St Francis whose name the pope adopted when he was elected in March.
There has been a giant mobilisation of Catholic churches worldwide -- through traditional homilies at mass as well as via social media -- for the faithful to pray for peace and engage in some form of fasting.
The four-hour prayer vigil in the Vatican starts at 1700 GMT but there will be similar smaller initiatives around the world from Baghdad to Jerusalem, from Mumbai to Buenos Aires, from Washington DC to Beirut.
The pope's call is not unprecedented but unusual and experts have likened it to the global day of prayer instituted by late pope John Paul II following the September 11, 2001 attacks on the United States.
It has also been compared to impassioned calls for peace by John Paul II in the run-up to the Iraq war and a speech by another papal predecessor, Paul VI, against the Vietnam War at the United Nations.
"This cry from the pope distills the calls coming from the one big family that is humanity," French cardinal Roger Etchegaray, who led peace missions on behalf of John Paul II ahead of the Iraq war in 2003, told AFP.
The head of the Vatican's Pontifical Council for Families, Monsignor Vincenzo Paglia, even wrote an open letter to parents asking them to take part.
"Do not be afraid to offer your children a sober meal," he said, adding: "Invite grandparents to a meal that will be poor in nourishment but rich in words."
"If anyone has experienced war, they should talk about what it means to live under a bombardment -- the uncertainty of what tomorrow will bring," Paglia said.
Brazilian cardinal Joao Braz de Aviz, head of the pontifical council overseeing Catholic orders on five continents, said churches and monasteries throughout the world should also take part in the prayers.
Among the many taking part in the initiative will be Bishop Theophile Kaboy in Goma in the war-riven Democratic Republic of Congo, who said he would hold a special prayer session in the local cathedral.
"We also ask that our people, who have lived with war for 20 years, not be forgotten," he said.
The appeal has been particularly well received in the Middle East, where Christian patriarchs who are often in competition with one another have been united in their concern about the possible escalation of Syria's civil war and the rise of radical Islamism.
"Peace is a good which overcomes every barrier, because it belongs all of humanity #prayforpeace," Pope Francis wrote in one tweet, with another saying simply: "Never again war! War never again!"
Francis is expected to be present for the duration of the marathon prayer vigil on St Peter's Square and will speak briefly in-between long moments of silence and recitals of invocations for peace.
A famous icon of the Virgin Mary will also be carried across the famous square by a group of Swiss Guards.
Couples from Syria, Egypt, Russia and the United States are also expected to take part in the ceremony, which will end with a solemn papal blessing.
Several Italian government leaders have said they will take part, after Italy said it will not take part in any armed intervention without a UN mandate.
Many non-Catholic groups including the far-left Left, Ecology and Freedom party, the anti-clerical Radical Party and the far-right European Solidarity Front for Syria have said they also support the prayer day.
© AFP 2014