During his general audience before meeting with the spouses Wednesday, the pope asked the faithful to pray for the summit attendees, the Italian news agency ANSA reported Thursday.
"This important world summit can lead to decisions useful for the progress of all peoples, especially the poorest," Pope Benedict XVI said.
All of the first ladies dressed in black and wore veils, except for South African President Jacob Zuma's wife, who wore traditional clothing, the news agency said.
U.S. first lady Michelle Obama did not attend the meeting because she and her husband, President Barack Obama, were to have a private audience with the pontiff Friday. France's first lady Carla Bruni did not arrive in Italy until Thursday. Japanese Prime Minister Taro Aso and his wife met the pope Tuesday.