More than two hours after the 115 cardinals withdrew into strict seclusion on Monday, black smoke emerged from a chimney on the Sistine Chapel signalling that the first vote had been inconclusive.
Initially, it appeared to the 20,000 people massed on Saint Peter's Square that the first wisps were white, indicating a new spiritual leader of the 1.1 billion Roman Catholics worldwide after the death of Pope John Paul II.
A roar went up, applause broke out, and the crowd surged forward - "It's white! It's white!" many shouted - only to quickly melt away when the puffs turned into thick, black smoke.
"It seemed white so we all ran forward toward St Peter's hoping to see the new pope," Nino Trifaro said, adding, "Then we realised."
Still, the 70-year-old Italian said, "it's an unforgettable moment."
John Paul II was the 265th pope
since St Peter
The prelates will not vote again until Tuesday, and from then on four times a day until they reach the required two-thirds consensus.
Earlier the cardinals locked themselves inside the Sistine Chapel to start the conclave to elect a successor to John Paul II.
Before the chapel door was closed on Monday, the cardinal electors swore an oath of secrecy and fidelity to the Church.
The door was closed at 5.27pm (1527 GMT) after the master of ceremonies, Archbishop Piero Marini, said "Extra Omnes" (Latin for "Everyone Out"), asking everyone not associated with the conclave to leave the frescoed room.
Left inside were the 115 cardinal electors, Marini and one other "outsider" - Czech Cardinal Tomas Spidlik, who was set to give a private sermon to the cardinals reminding them of the gravity of their responsibility.
After the sermon by Spidlik, who is 85 and therefore not eligible to remain in the conclave, both he and Marini had to leave the room, allowing the conclave proper to begin.
As has happened before, the deadlock may force the cardinals to look in unexpected directions.
This apparently was the case in 1978, when John Paul II was selected.
The cardinals are divided by origin and ideological affiliation, each reflecting a different wing within the Catholic Church.
Getting the required majority to elect a new pope may take up to several days and repeated voting. Since the death of John Paul II, here are four of the most talked about contenders:
Among developing world conservatives, Francis Arinze, a 72-year-old Nigerian, is considered a genial but shrewd Vatican insider and renowned for his ability to say tough things with a smile.
Intransigent on matters of life and sex, he was born an animist in east Nigeria, becoming a teacher after his studies. In 1965, at 32, he was consecrated the world's youngest Roman Catholic bishop.
During his 18 years as archbishop of Onitsha, the number of Catholics in his charge is said to have increased by 65%.
In 1985, John Paul called him to Rome to head the inter-faith body that maintains dialogue with other religions, and Arinze has been specialising in Islamic-Christian relations for the last 20 years.
In 2003, he was promoted to one of the Church's top jobs, overseeing liturgy.
Oscar Rodriguez Maradiaga
Among developing world liberals, the main example is Oscar Rodriguez Maradiaga, 62. The Honduran archbishop has remained in the mainstream on doctrine while espousing a social policy, which sometimes echoes secular critics of globalisation.
Among his most famous soundbites: "Often justice comes only for people who are rich," and, "We are never going to have peace while poverty is increasing."
He speaks six languages, plays five musical instruments and has degrees in psychology and theology. He learned English from aviation manuals while studying for a pilot's licence.
If the cardinals eventually opt for a European bridge-builder, they could turn to Christoph Schonborn, a 60-year-old Austrian. The archbishop of Vienna has shown a talent for reconciliation.
He took over an archdiocese, traumatised by claims of sex abuse and swiftly restored morale of the faithful.
The son of refugees, who fled communist Czechoslovakia, he is an ardent supporter of John Paul's view that Europe must pay equal attention to east and west Christianity, breathing "with two lungs".
Having entered the Dominican order at 18, he is considered an outstanding scholar and theologian, who has been touted as the replacement for Joseph Ratzinger, the guardian of doctrinal orthodoxy.
If the cardinals go for the bureaucrat, who can master Vatican politics, then many think Angelo Sodano - an Italian and currently secretary of state (equivalent to a prime minister in the Roman Curia) the central administration of Roman Catholicism - is the one.
The 77-year-old twice pushed his name into the headlines recently when he raised the issue of the late pope's possible retirement and when he worked a reference to "John Paul the Great" (thus in papal politics, beginning the beautification to sainthood) into the requiem service.
The son of an Italian politician, he became a hate figure for the left when he served as an uncritical envoy to Chile's military junta in the 1970s.
Whoever the cardinals choose, perhaps the old adage ''a fat pope usually follows a thin pope'' may be the best indicator of who the next bishop of Rome will be.