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 at a time of strife and scandal for the Roman Catholic Church.

Thousands of faithful huddled in St. Peter's Square to watch the smoke pour out of the narrow flue in the rain-laden gloom.

The smoke showed that the 115 cardinal electors aged under 80 had cast their first round of ballots in the conclave to select a new pontiff. After the ballots are cast they are burned, with flares added to produce coloured smoke to signify the result to people waiting outside. When a pope has been elected, the smoke will be white.

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

Vatican insiders say Italy's Angelo Scola and Brazil's Odilo Scherer have emerged as the men to beat. The former would bring the papacy back to Italy for the first time in 35 years, while the latter would be the first non-European pope in 1,300 years.

Cardinals will hold four ballots a day from Wednesday until one man has won a two-thirds majority - or 77 votes. The smoke signals are expected at around noon and 7 p.m each voting day. However, smoke could emerge earlier if the new pontiff is elected in the first ballot of one of the sessions.

As in medieval times, the cardinals will be banned from communicating with the outside world. The Vatican has also taken high-tech measures to ensure secrecy in the 21st century, including electronic jamming devices to prevent eavesdropping.

Presented by Adam Justice