VATICAN CITY (Reuters) - Thick black smoke billowed into the night sky from the Sistine Chapel's chimney on Tuesday, signalling an inconclusive first vote in the conclave to elect a new Pope.

Thousands of faithful huddled in St. Peter's Square 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 then 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 this week that it might take four or five days to pick the man to replace Pope Benedict, 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 on Wednesday to continue their votes, 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.