The American Pope, who is fluent in Spanish after years serving as a missionary and bishop in Peru, received an enthusiastic response when he used Catalan, the regional language of north-eastern Spain's Catalonia region on Tuesday.
Leo's remarks in Catalan followed criticism of an initial Vatican statement that he would speak only Spanish during the visit. Barcelona Archbishop Juan José Omella had said the Pope would try his best in response to calls for him to speak Catalan.
The Pope's command of the regional language appeared to exceed the expectations of many.
During the vigil, young participants asked Leo questions on issues including the search for meaning beyond career ambitions, depression and faith, and family difficulties. The Pope responded in both Catalan and Spanish.
Earlier, Leo blessed 30 ambulances destined for Ukraine.
The visit has drawn large crowds since the Pope arrived in Spain on Saturday.
Leo, elected Pope in May 2025, has gained support for messages emphasising peace, international law and human dignity, including the rights of migrants and refugees.
He has also warned against growing political polarisation and right-wing populist trends.
More than 1.2 million people attended an outdoor mass celebrated by the Pope in Madrid earlier in the visit.
A mass at Barcelona's landmark basilica, the Sagrada Família, is expected to be the highlight of his stay in the city on Wednesday evening.
Leo will then travel to Spain's Canary Islands, where he is due to meet migrants who have arrived from Africa via one of Europe's most dangerous migration routes.
