VATICAN CITY – The gathering of Roman Catholic cardinals to pick a new pope began Tuesday with an oath of secrecy and an inaugural vote that produced no quick winner but gave the prelates their first look at which candidates are garnering the most support.

Black smoke billowed from the chimney atop the Vatican’s Sistine Chapel on Tuesday evening less than 2-1/2 hours after the doors were shut to outsiders and the cardinals within prepared to cast ballots for the next leader of the world’s 1.2 billion Catholics. The dark fumes signified an inconclusive vote, and a disappointed murmur rose from the crowd of thousands in St. Peter’s Square.

An immediate agreement on a new pontiff was never likely. But the first vote is always important as a measure of the strength of support for particular candidates. It also lets the cardinals chat and caucus at dinner and sleep on the result, to decide in the morning whether to stick to their original choice, switch sides or look for dark-horse alternatives.

What happens in the Sistine Chapel is supposed to stay in the Sistine Chapel; even support staffers attending the 115 cardinals have been sworn to silence on pain of excommunication. But attention before Tuesday focused on two contenders, Angelo Scola of Italy and Odilo Pedro Scherer of Brazil, as the strongest candidates going into the conclave.

Whoever is chosen by at least a two-thirds majority — 77 votes in this case — will become the 266th pope, succeeding Benedict XVI, whose resignation last month threw the Catholic Church into uncertainty. He is the first pontiff to step down in six centuries, and leaves behind an institution weakened by factionalism in the Vatican hierarchy, a sex abuse scandal and challenges from other religions and secularism.

“You feel Rome is without a shepherd,” said Maria Vrede Timmermans, 35, a nun from the Netherlands who waited with her sisters in St. Peter’s Square for the chimney to pour forth news of the first vote’s outcome. “We are watching the Holy Spirit working, and it is hopeful and beautiful.”

The red-hatted cardinals, “princes” of the church, filed into the Sistine Chapel in a solemn, chanting procession Tuesday afternoon, mindful of the responsibility on their shoulders.

After bowing to the altar in front of Michelangelo’s magnificent fresco “The Last Judgment,” they took their places in order of seniority at tables on either side. One by one, they pledged to keep the proceedings in strictest confidence, “so help me God and these holy Gospels which I touch with my hand.”

A prelate then ordered the others to leave. Video from inside the chapel aired by the Vatican’s television service, which had beamed images of the centuries-old ritual around the world, was cut.

Vatican watchers expect a pope to be named within two or three days. No conclave of the past century has lasted more than five days. Red velvet drapes have already been hung on the central balcony of St. Peter’s Basilica, where the new pope will be revealed with great fanfare to the expected cheering pilgrims below in his first public appearance.

The field of possible candidates is unusually wide this time, amid reports of opposing camps forming among those who want a new leader to shake up the Vatican administration and others who are looking for a more pastoral figure. A week of meetings allowed the cardinals to hash out issues facing the church and to size up one another as potential occupants of the throne of St. Peter.

At a pre-conclave Mass on Tuesday morning in the cavernous basilica, Cardinal Angelo Sodano exhorted his fellow prelates to show unity and to rally around whoever is elected.

“St. Paul teaches that each of us must work to build up the unity of the church,” said Sodano, 85.

Sodano, who because of an age limit is not eligible to join the conclave, prayed for divine wisdom for the “cardinal electors.”

The conclave will hold up to four rounds of voting in a day, two in the morning and two in the afternoon. No campaigning, overt or subtle, is allowed in the chapel, where ballots are to be cast in an atmosphere of prayerful consideration and dignified reverence.

Church politics were far from the mind of Andrew Linbeck, an asset manager from Houston who flew in with his wife and three daughters for the conclave and was waiting in St. Peter’s Square Tuesday evening. “Our flights home are Saturday,” he said. “Hopefully we’ll hear ‘Habemus papam’ by then,” referring to the Latin declaration — “We have a pope.”