Pope Francis has arrived in Athens for the first visit to the Greek capital by a pontiff in two decades.

The pope's trip will see him return on Sunday to the island of Lesbos, which he last visited in 2016 during the early years of the migration crisis.

But his visit to Athens is the first by a pope since John Paul II in 2001, which in turn was the first papal visit to the city since the 1054 Schism between the Catholic and Orthodox Churches.

Pope Francis is seeking to improve historically difficult relations with the Orthodox Church while also highlighting the plight of thousands of refugees and asylum seekers in Greece.

"I ardently long to meet you all, all, not only Catholics, but all of you," he said in a message before embarking on his trip, which began on Thursday with a two-day visit to Cyprus.

"By meeting you, I will quench my thirst at the springs of fraternity," he said.

Pope Francis will meet Greek President Katerina Sakellaropoulou, Prime Minister Kyriakos Mitsotakis and the head of the Church of Greece Archbishop Ieronymos.

He is then scheduled to see members of Greece's small Catholic community.

"The Holy Father's presence in Greece is a boost for us... Catholics in Greece must take advantage of it," Markos Foscolos, parish priest of St Nicholas on the island of Tinos, told reporters this week.

Catholic numbers have been boosted in recent years, with between 50,000 and 60,000 local Catholics being joined by another 250,000 from the Philippines, Poland and African countries.

Up to 2,000 police will be deployed in Athens to monitor possible disruptions by Orthodox hardliners, which blame Catholics for the Schism and the 1204 sacking of Constantinople during the Fourth Crusade.

"They will be few, but loud," noted Petros Panagiotopoulos, a theologian at Aristotle University in Thessaloniki.

Relations with the Church of Greece are much better than they were ahead of John Paul's visit, Pierre Salembier, head of the Jesuit Catholic community in Greece, told AFP.

But he said there were still some "known anti-Catholic fanatics" within the Church's governing body.

The bishop of Piraeus called the pope's visit "immoral", according to the union of Orthodox journalists.

In Cyprus yesterday, Francis condemned "slavery" and "torture" in migrant camps, drawing parallels with World War II.

The Cyprus government said 50 migrants, including two Cameroonians stuck for months in the divided island's buffer zone, will be relocated to Italy thanks to Pope Francis.

Tomorrow, the pope will again visit Greece's Lesbos, a flashpoint of the 2015 refugee crisis and thereafter, "as a pilgrim to the wellsprings of humanity" to call for the integration of refugees.

The sprawling and dismal camp of Moria, which the pontiff visited in 2016, burnt down last year and has been replaced by a new EU-funded facility.

The new Mavrovouni camp is part of a series of "closed" facilities on Greek islands with barbed wire fencing, surveillance cameras, X-ray scanners and magnetic gates that are closed at night.

NGOs and aid groups have raised concerns about the new camps, arguing that the movement of people should not be restricted.

Thirty-six groups active in Greece this week sent a letter to the pope, drawing his attention to the rights of people in the camps and asking for his help in halting illegal pushbacks of migrants allegedly carried out by Greek border forces.

The pontiff is expected to visit the camp with senior officials from Greece and the EU and will meet two "randomly chosen" families, an official said.

"We await him with open arms," said Berthe, a Cameroonian asylum seeker at the camp.

She said she hoped the pope "will pray for us to help us overcome the insecurities we have lived, through faith."

On Wednesday, nearly 30 asylum seekers landed near the camp. Yesterday, two migrants died when a speedboat overturned near the Greek island of Kos.