Pope Francis on Sunday described the war in Ukraine as a "macabre regression of humanity" that makes him "suffer and cry."

Speaking to thousands of people crowded into St. Peter's Square at the Vatican, the pope called for humanitarian corridors to be opened to evacuate civilians trapped inside or near a steel plant in the Ukrainian port city of Mariupol.

Evacuation of civilians at the Azovstal steel plant, where Ukrainian forces have been staging a last stand against Russian troops, have started, Ukrainian President Volodymyr Zelenskyy, the Red Cross and the Russian Ministry of Defense confirmed on Sunday.

During Sunday's Vatican service, Francis repeated his criticism of Russia for invading Ukraine.

"My thoughts go immediately to the Ukrainian city of Mariupol, the city of Mary, barbarously bombarded and destroyed," the pontiff said of the Russian-controlled southeastern port city, which is named after Mary. "I suffer and cry thinking of the suffering of the Ukrainian population, in particular the weakest, the elderly, the children."

In Catholicism, the month of May is dedicated to Mary, the mother of Jesus. Francis asked for monthlong prayers for peace in Ukraine.

"While we are witnessing a macabre regression of humanity, I ask you, together with so many anguished people, if we are really seeking peace, if there is the will to avoid a continuous military and verbal escalation, if we are doing everything possible to make the weapons stop? Please, let us not give in to the logic of violence, to the perverse spiral of arms. Let us take the path of dialogue and peace. Let us pray."

-ABC News' Rashid Haddou

Some civilians who have been holed up in houses near fighting being waged at a steel plant in the Ukrainian city of Mariupol were evacuated via a humanitarian corridor established on Saturday and Sunday, according to the Russian Ministry of Defense.

At least 46 people were allowed out of the heavily bombed port city when they emerged from houses near the Azovstal steel plant, where Ukrainian forces have been staging a last stand against Russian troops ringing the facility, the Russian Ministry of Defense said Sunday.

The civilians emerged from their houses during a period of calm in the fighting at the steel plant, according to the Russian military.

"Twenty-five local residents came out in the afternoon of April 30. At nightfall, taking the advantage of the regime of calm, a second group of 21 people came out and was taken to the populated locality of Bezimenne," the Russian Defense Ministry said.

The two groups of civilians came out "as a result of establishing a regime of calm and opening a humanitarian corridor from the residential houses nearest to the grounds of the Azovstal integrated steel works," the defense ministry's statement said.

Ukrainian President Volodymyr Zelenskyy confirmed on Twitter Sunday that the "evacuation of civilians from Azovstal began." Zelenskyy said about 100 civilians were evacuated from the plant on Sunday and that the U.N. is working on evacuating others.

A Red Cross representative told the Associated Press at least 20 civilians were evacuated from the Azovstal steel plant amid a temporary cease-fire observed by Ukrainian and Russian forces. A journalist for the AP reported seeing women and children arriving in Bezimenne, which is controlled by Russian-backed separatists in eastern Ukraine.

The AP reported the newly arriving evacuees were placed in a refugee tent camp and are expected to be escorted to the Ukrainian-controlled city of Zaporizhzhia by U.N. and Red Cross vehicles.

Several previous attempts to establish humanitarian corridors out of Mariupol have failed. Ukrainian officials claimed the previous evacuation were not successful because Russian troops did not follow through with the planned ceasefires.

House Speaker Nancy Pelosi this weekend told Ukrainian President Volodymyr Zelenskyy that additional U.S. support "is on the way."

"We are here to say to you that we are with you till this fight is over," Pelosi said during their meeting in Kyiv, according to a video released by her office on Sunday.

Pelosi, who traveled with a congressional delegation, is the highest-level U.S. official to visit Ukraine since Russia invaded on Feb. 24. Her delegation’s trip was previously unannounced.

"Our delegation traveled to Kyiv to send an unmistakable and resounding message to the entire world: America stands firmly with Ukraine,” Pelosi said in a statement after the meeting.

“When we return to the United States, we will do so further informed, deeply inspired and ready to do what is needed to help the Ukrainian people as they defend democracy for their nation and for the world,” Pelosi said.

House Speaker Nancy Pelosi led a delegation of representatives to meet with Ukrainian President Volodymyr Zelenskyy in war-torn Kyiv.

“We believe that we are visiting you to say thank you for your fight for freedom,” Pelosi said in a video posted by Zelenskyy on social media early Sunday morning.

Mariupol has not yet fallen, despite Russia's demands that Ukrainian troops defending the besieged Ukrainian port city surrender, according to Ukrainian Prime Minister Denys Shmyhal.

"There [are] still our military forces, our soldiers, so they will fight until the end," Shmyhal told ABC News' George Stephanopoulos in an exclusive interview Sunday on "This Week."

Mariupol is a strategic city for Moscow because it would allow Russian forces in the south to connect with troops in eastern Ukraine's contested Donbas region. It would also give Moscow a key port.

Although Mariupol remains under the Ukrainian government's control, Shmyhal said the city's residents are suffering.

"They have no water, no food, no heat, no electricity," he said. "They ask all of our partners to support and help stop this humanitarian catastrophe."

-ABC News' Monica Dunn