Pope Francis prayed for an end to the persecution of Christians in his Easter Sunday address, commemorating the students massacred by Islamist militants at Garissa University in Kenya.

Francis, after saying Mass for thousands of people in a rainy St. Peter's Square, delivered a mostly somber and grim "Urbi et Orbi" (to the city and the world) message.

Attacks on Christians in Africa and the Middle East have been the grim backdrop of all Holy Week ceremonies leading up to Easter.

"We ask Jesus, the victor over death, to lighten the sufferings of our many brothers and sisters who are persecuted for his name, and of all those who suffer injustice as a result of ongoing conflicts and violence - and there are many," he said.

The pope spoke as churches in Kenya, where al Shabaab gunmen massacred nearly 150 people, singling out Christians for point-blank executions, turned to armed guards to protect their congregations on the most important day of the Christian liturgical year.

"May constant prayer rise up from all people of goodwill for those who lost their lives – I think in particular of the young people who were killed last Thursday at Garissa University College in Kenya - for all who have been kidnapped, and for those forced to abandon their homes and their dear ones."

The 78-year-old Argentine pope, celebrating the third Easter of his pontificate, spoke from the central balcony of St. Peter's Basilica after saying a Mass below for tens of thousands of people wearing plastic ponchos and holding umbrellas against the driving rain.

Calling for peace in Libya, where last February Islamic State militants beheaded 21 Egyptian Coptic Christians, the pope called for an end to "the present absurd bloodshed and all barbarous acts of violence".

He prayed for peace in Syria, Iraq and Yemen, Sudan, South Sudan, the Democratic Republic of Congo and Nigeria, where Boko Haram Islamist militants have also targeted Christian churches.

"We ask for peace and freedom for the many men and women subject to old and new forms of enslavement on the part of criminal individuals and groups," he said.

"Peace and liberty for the victims of drug dealers, who are often allied with the powers who ought to defend peace and harmony in the human family. And we ask peace for this world subjected to arms dealers, who make their money from the blood of men and women" he said.

Just about the only positive part in the pope's address was a reference to the deal reached in Switzerland last week between Iran and the international community on a framework for a nuclear accord.

"In hope we entrust to the merciful Lord the framework recently agreed to in Lausanne, that it may be a definitive step toward a more secure and fraternal world," he said.