Pope Francis on Sunday sent prayers to the victims of the concert hall attack in Moscow, calling the violence "inhuman acts" that offended God.

"I assure my prayers for the victims of the cowardly terrorist attack carried out the other evening in Moscow," said the 87-year-old pope, following a Palm Sunday mass in Saint Peter's Square.

"May the Lord welcome them in his peace and comfort their families, and convert the hearts of those... who organise and carry out these inhuman actions which offend God, who commanded 'Thou shalt not kill'."

The pontiff presided over Palm Sunday Mass in front of a crowd the Vatican estimated at 25,000 people, but spoke in a weak, faltering voice and did not deliver his homily as planned.

He appeared in good spirits following mass, however, circling the piazza, smiling and waving at the crowd from his vehicle. The pontiff caught flu last month and has subsequently asked others to read his texts at appearances.

Francis has suffered a number of health issues in recent years, from knee and hip pain to an inflamed colon and hernia surgery last year.

Sunday's liturgical celebration marked the start of Holy Week, in which the pope presides over a series of important events leading up to Easter, Christianity's most important holiday.

Last year, for the first time, Francis missed the traditional "Way of the Cross" prayer service on Good Friday.

The weather at the time was unusually cold and he was recovering from a bronchitis infection that saw him hospitalised for three nights. (AFP)