Thick black smoke billowed from the Sistine Chapel's chimney signalling an inconclusive first vote in the conclave to elect a new pope at a time of strife and scandal for the Catholic Church.

Thousands of followers huddled in St Peter's Square in Rome to watch the smoke pour out of the narrow flue in the rain-laden gloom following a day rich in ritual and pageantry.

After praying for divine guidance, the red-hatted cardinals took a solemn vow in Latin never to divulge any details of their deliberations. They secluded themselves behind the chapel's heavy wooden doors.

No conclave in the modern era has chosen a pope on its first day, and some cardinals speculated it might take four or five days to pick the man to replace Pope Benedict, 85, who unexpectedly abdicated last month.

The so-called "Princes of the Church" will spend the night in a Vatican hotel before returning to the frescoed Sistine Chapel this morning to continue voting, with two rounds set for the morning and two for the afternoon.

Until they choose a new pontiff, their only communication with the outside world will be the smoke from the Chapel chimney – black when voting sessions end with no result and white when a pontiff is elected.

The crowd's excited cheers when the first puffs of smoke emerged turned to disappointed sighs when they saw it was signalling no surprise early decision.

"I am on vacation and can't believe how lucky I am to be here at this moment," said Patricia Purdy, a retired teacher from New York, adding it was time for a younger pope.

"It would be good if he was young, so he can relate to younger people and bring them closer to the Church."

The 266th pontiff in the Church's 2000-year history will face a daunting array of problems, including sex abuse scandals, infighting within the Vatican bureaucracy and the spread of secularism.

No clear-cut front-runner has emerged, with some prelates pushing for a strong manager to control the central administration, the Curia, while others want a powerful pastor to promote their faith across the globe.

Italy's Angelo Scola and Brazil's Odilo Scherer are spoken of as strong contenders.