Pope Francis on Wednesday called for “unanimous condemnation” of extremist attacks in Brussels and elsewhere and urged prayers to convert hearts “ blinded by cruel fundamentalism.”

At the end of his weekly public audience, the pontiff led thousands of people in St. Peter’s Square in silent prayer for the victims of the airport and metro attacks a day earlier in the Belgian capital.

He told the crowd that he assured the “dear Belgian people” of his prayers and closeness.

“I yet again appeal to all persons of good will to unite in unanimous condemnation of these cruel abominations that are causing only death, terror or horror,” the pope said.

He asked people to keep praying, to comfort those suffering, as well as to “convert the hearts of these people blinded by cruel fundamentalism.”

The U. S. Embassy in Rome Wednesday issued a travel alert advising “particular caution during religious holidays” as well as at large gatherings.

Holy Week ceremonies over the next few days are expected to draw large crowds. On Friday night, Francis will preside over a Way of the Cross procession at the Colosseum. On Sunday, he will celebrate Easter Mass in the square and give a blessing from the central balcony of St. Peter’s Basilica, an event which in the past has drawn some 100,000 faithful.