A total of 133 cardinals from around the world, including four Poles, began deliberations behind closed doors earlier in the day to choose a successor to Pope Francis, who died last month.
Thousands of faithful gathered in St. Peter's Square to await the traditional signal—smoke emerging from the chapel's chimney—to indicate whether a decision had been reached.
The black smoke appeared more than three hours after the conclave began, confirming that no candidate had received the required two-thirds majority to become the next leader of the 1.4-billion-strong Church.
White smoke will signal a successful election.
However, a decision on the first day was not expected; a pope has not been chosen on the initial ballot in modern times, the Reuters news agency reported.
Ahead of the conclave, some cardinals voiced hope that a new pontiff could be elected on Thursday or Friday to demonstrate unity following the often-divisive, 12-year papacy of Francis, according to Reuters.
Voting will resume on Thursday, with two rounds scheduled in the morning and two in the afternoon.
The conclave will continue until one cardinal secures at least 89 votes, Polish state news agency PAP reported.
Polish cardinal Konrad Krajewski, a close ally and charity envoy of Pope Francis, has been mentioned by some media outlets as a potential candidate.
Francis, who was elected in 2013 following the resignation of Pope Benedict XVI, died on April 21 at the age of 88.
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Source: IAR, PAP, Reuters, Vatican News