Pope Leo XIV calls for end to wars in Ukraine, Gaza

Cardinal Giovanni Battista Re, the dean of the College of Cardinals, reminded his fellow cardinals on Wednesday that they were about to undertake "an act of the highest human and ecclesial responsibility" in voting for a new pope.

"This is a human act for which every personal consideration must be set aside, keeping in mind and heart only the God of Jesus Christ and the good of the Church and of humanity," Re said in a homily during a Mass in St. Peter's Basilica.

But, he said, the election of a new pontiff also comes at a "difficult and complex turning point in history."

"Today's world expects much from the Church regarding the safeguarding of those fundamental human and spiritual values without which human coexistence will not be better nor bring good to future generations," Re said.

The Cardinals' Mass began in St Peter’s Basilica on Wednesday morning ahead of the papal conclave.

The mass, which was being broadcast live around St. Peter's Square was expected to last until 11:30 a.m. local time.

Later on Wednesday, the 133 voting cardinals will enter the Pauline Chapel at approximately 4:15 p.m. before a procession into the Sistine Chapel where they take the oath at around 4:30 p.m.

Of all the ceremonies associated with electing a new pope, the one most familiar to the general public is the smoke that emanates from a stovepipe chimney atop the Sistine Chapel after every round of balloting.

Black smoke -- fumata nera in Italian -- indicates an inconclusive vote, while white smoke -- fumata bianca -- will signify that a new pope has been elected. Along with the white smoke, the bells of St. Peter's Basilica will ring to proclaim the event to the world.

The ceremonial smoke is not created by the burning of the ballots. The voting slips are burned in a stove that dates back to the 1922 conclave and is set up for the occasion in the Sistine Chapel.

The smoke that wafts from the stovepipe chimney is created using chemical pellets that are burned in another stove that's connected to the chimney, which is temporarily erected atop the Sistine Chapel just for that purpose.

Assuming the elected cardinal accepts the office, the new pope's identity is typically revealed within an hour of the final ballot, after he chooses the name by which he will be known as pope.

The new pope then emerges onto the balcony to present himself to the world and deliver his first blessing to the crowd gathered below in St. Peter's Square.

-ABC News' Christopher Watson

As Vatican City begins to fill with the faithful on Wednesday morning, the weather forecast calls for temperatures in the mid-60 degrees Fahrenheit, with midday showers expected, according to the Italian Meteorological Service.

More than 25,000 people are expected to gather into St. Peter's Square on Wednesday, along with crowds of people spilling over into the nearby roads.

About 4,000 security personnel are on hand, along with about 1,000 volunteers. And about 5,300 journalists from around the world will be present, waiting for news from the conclave.

-ABC News' Joe Simonetti