Catholics worldwide held a day of fasting and prayer for peace in Syria joined by Jews, Muslims and Orthodox Christians, with Pope Francis set to host a mass vigil on Saturday.
Francis has called for a "cry for peace" to rise up around the globe and has said he will attend the four-hour prayer session in St Peter's Square.
Earlier in the week he wrote to leaders of the G20 leading world economies urging them to "lay aside the futile pursuit of a military solution".
The Vatican has warned military strikes planned by France and the United States could escalate the conflict and cause it to spill over into the region.
The Catholic Church, which counts 1.2 billion faithful, has mobilised to adhere to the pope's call through homilies in churches as well as through social media.
"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!"
The Vatican has even issued instructions for Catholic parents to prepare "sober" family meals with children and grandparents that would be "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," said Vincenzo Paglia, head of the Pontifical Council for Families.
"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.
When he announced the initiative on Sunday, Francis urged Christians from other denominations, faithful from other religions as well as atheists to join in.
Syria's Sunni Muslim leader, Grand Mufti Ahmed Badreddin Hassoun, called for Syrians to join in the prayers and the patriarch of Constantinople, Bartholomew I, spiritual leader of the world's Orthodox, also backed the call.
In France, Muslim faithful at the Great Mosque of Paris held prayers on Friday following the pope's call and asking "for the blood to stop flowing" in Syria.
Chief Rabbi of Rome Riccardo Di Segni said the Jewish community was "in harmony" with the Vatican and would focus their prayers on the plight of Syria.
In Lebanon, the vice president of the Shiite Higher Council, Sheikh Abdel Amir Qabalan, voiced support.
"Islam calls consistently for peace and harmony, and we consistently condemn killings, terrorism and foreign interference," he said.
The appeal has been particularly well received by Christian minorities in the Middle East, where leaders 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.
Traditionally anti-clerical groups, like the Radicals and the Left, Ecology and Freedom party in Italy, have also said they are supporting the pope's appeal.
Several Italian government leaders have said they will take part, after Italy said it will not participate in any armed intervention without a United Nations mandate.
The prayer vigil in the Vatican starts at 1700 GMT but similar smaller initiatives are being held in places of worship around the world from Baghdad to Jerusalem, from Mumbai to Buenos Aires, from Washington DC to Beirut.
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.
Francis is expected to be present for the duration of the vigil and will speak briefly in between long moments of silence and recitals of invocations for peace.
An 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 expected to take part in the ceremony, which will end with a solemn papal blessing.
The pope's call is not unprecedented -- previous popes have appealed against the Iraq war, the conflicts in the Balkans and the Vietnam War -- but it is rare and unusual.
The last time the Vatican called a similar day of prayer and fasting was late pope John Paul II in the wake of the September 11, 2001 attacks on the United States.