The cardinals began the heavily ritualised and secret process of choosing a new leader for the world's 1.4 billion Catholics on Wednesday, locked away in complete isolation from the world.
In the evening, black smoke billowed from a chimney on the roof of the chapel, signalling an inconclusive first ballot.
On Thursday, they were expected to begin voting from about 9am (5pm AEST), according to the Vatican.
White smoke would signal the election of a new church leader to replace Francis, who died in April.
If the first vote on Thursday is decisive, the election could be announced as early as 11am.
If not, smoke signals would come later in the day.
No pope in modern times has been elected on the first attempt so Wednesday's black smoke was widely expected, but given recent history, a final result is possible from the second day, when up to four rounds of voting can take place.
A record 133 cardinals from 70 countries are involved in the ballot, up from 115 from 48 nations in the last conclave in 2013 - growth that reflects efforts by Francis to extend the global reach of the church.
A record 133 cardinals from 70 countries are involved in the ballot, up from 115 from 48 nations in the last conclave in 2013 - growth that reflects efforts by Francis to extend the global reach of the church.
Argentine-born Francis was elected in 2013 at the end of the second day, after five rounds of voting. In 2005 it took two days but only four votes to pick Germany's Benedict XVI.
While no clear favourites have emerged, Italian Cardinal Pietro Parolin, who served as the Vatican's number two under Francis, and Filipino Cardinal Luis Antonio Tagle are considered the front-runners.
If it becomes obvious that neither can win, votes are expected to shift to other contenders, with the electors possibly coalescing around geography, doctrinal affinity or common languages.
Other potential "papabili" - papal candidates in Italian - are France's Jean-Marc Aveline, Hungary's Peter Erdo, American Robert Prevost and Italy's Pierbattista Pizzaballa.
During the conclave, cardinals are sequestered from the world and sworn to secrecy, their phones and computers confiscated, while they are shuttled between the Sistine Chapel for voting and two Vatican guesthouses to sleep and dine.
In recent days, they have offered different assessments of what they are looking for in the next pope. Francis's relatively liberal pontificate was marked by bitter divisions between traditionalists and modernisers.
While some urged for continuity with his vision of greater openness and reform, others longed to turn the clock back and embrace traditions. Many have indicated they want a more predictable, measured pontificate.
