Vatican City, Holy See - Thick black smoke emerged Wednesday from the chimney of the Sistine Chapel in a sign that cardinals had failed to elect a new head of the Catholic Church in their first conclave vote.
Tens of thousands of people gathered in St Peter's Square to await the smoke, which came around three hours and 15 minutes after the 133 cardinals were closed in.
The prelates will now withdraw to the Santa Marta guesthouse where they are staying for the election, before starting to vote again on Thursday.
Cardinals were called back to Rome following the death of Pope Francis on April 21 after 12 years as head of the world's 1.4 billion Catholics.
Under a centuries-old ritual, those aged under 80 vote in secrecy in the Sistine Chapel until one of them secures a two-thirds majority -- 89 votes -- to be elected pope.
Locked away to avoid distraction, their only means of communicating the outcome is by burning their ballots with chemicals to produce smoke.
It is black if there is no decision, white if they have a new pope.
This conclave is the largest and the most international ever, assembling cardinals from around 70 countries -- many of whom did not previously know each other.
There is no clear frontrunner to succeed the charismatic Argentine Francis, with the cardinals representing a range of progressive and conservative traditions within the Church.
But the challenges facing the 2,000-year-old institution are clear.
The new pope will have to face diplomatic balancing acts at a time of geopolitical uncertainty, as well as deep splits within the Church.
There is also the continued fall-out from the clerical child abuse scandal and -- in the West -- increasingly empty pews.
- Waiting for the smoke -
The start of the conclave, with a solemn procession of cardinals and other clergy into the Sistine Chapel, was streamed live on large screens in front of St Peter's Basilica.
As night fell, the crowds swelled, including many young people and children. Some sang hymns while others danced -- while one young woman showed off her black miniature poodle dressed as the pope.
The screens went black at 9 pm (1900 GMT), eliciting groans from the crowds, minutes before the black smoke was spotted wafting from the chimney -- bringing on a new wave of disappointed reaction.
Still, James Kleineck, 37, from Texas, said he was "excited" to witness the unique event.
"I don't mind that it's black smoke, it shows the Holy Spirit is at work. There will be other votes soon enough, we will get our pope," he told AFP.
The cardinal electors had earlier gathered in the nearby Pauline Chapel in silent prayer before proceeding to the 15th-century Sistine Chapel, where tables and chairs were laid out beneath Michelangelo's frescoes.
They took a group oath of secrecy before each cardinal approached the altar to utter his personal vow not to reveal what happened in the conclave, on pain of excommunication.
According to a video feed produced by the Vatican, they filed up one by one in front of the Renaissance master's depiction of the Last Judgement, when Christians believe God determines who goes to heaven or hell.
The conclave's master of ceremonies then declared "Extra omnes" -- "Everybody out" in Latin -- and then shut the heavy doors of the chapel.
There are four votes a day until a winner is elected.
Both Francis and his predecessor Benedict XVI were elected within two days, but the longest papal election lasted 1,006 days, from 1268 to 1271.
- Pray for unity -
The cardinals joined a mass in St Peter's Basilica ahead of the conclave on Wednesday morning, where Cardinal Giovanni Battista Re, dean of the College of Cardinals, offered some final advice.
"We are here to invoke the help of the Holy Spirit, to implore his light and strength so that the pope elected may be he whom the Church and humanity need at this difficult and complex turning point in history," he said.
"This is also a strong call to maintain the unity of the Church... a unity that does not mean uniformity, but a firm and profound communion in diversity."
He called it a choice of "exceptional importance", requiring the red-robed prelates to set aside "every personal consideration".
Battista Re himself is too old to vote.
The mass was the last rite to be celebrated publicly before the 267th pope is presented to the world from a balcony of St Peter's Basilica.
- Women protest -
The cardinals have spent days discussing the most pressing challenges facing the Catholic Church and the character traits its new leader needs.
Burning issues include falling priest numbers, the role of women, the Vatican's troubled balance sheets and how to adapt the Church to the modern world.
Battista Re urged cardinals to pray for "a pope who knows how best to awaken the consciences of all... in today's society, characterised by great technological progress but which tends to forget God".
Meanwhile across Rome, women's rights activists gathered to protest the absence of women in the conclave.
"We are saying to the cardinals, you cannot keep ignoring 50 percent of the Catholic population," said Miriam Duignan, of the UK-based Wijngaards Institute for Catholic Research.
Some 80 percent of the cardinals voting were appointed by Francis -- an impulsive yet charismatic champion of the downtrodden.
But while cardinals have said they would favour a leader able to protect and develop his legacy, others want a more conservative defender of doctrine.
More than a dozen names are circulating, from Italian Pierbattista Pizzaballa to Hungary's Peter Erdo and Sri Lanka's Malcolm Ranjith.
