Pope Leo XIV on Saturday denounced the widening gap between the haves and have-nots as he visited Monaco, a millionaires' playground that was the surprise pick for the first western European trip of his papacy.

Arriving by helicopter from Rome as the first pope to visit the principality in nearly half a millennium, Leo was greeted by Monaco's ruler Prince Albert II and his wife, Princess Charlene, at Monte Carlo's heliport.

Just after his arrival at the tiny principality on the French Riviera, the pope condemned what he termed the widening "chasms between the poor and the rich".

In an address in French from the balcony of the Prince's Palace, the first US-born pontiff launched into an address which touched on the themes of social justice and inequality dear to his predecessor, Pope Francis.

He denounced the "unjust configurations of power, structures of sin that dig chasms between poor and rich, between the privileged and the rejected, between friends and enemy".

And within walking distance of Monaco's casinos, Leo insisted that wealth should serve "law and justice, especially at a historical moment when displays of force and the logic of omnipotence wound the world and jeopardise peace" -- a clear reference to the growing number of conflicts across the globe.

"To live here is for some a privilege and for everyone a specific call to question their own place in the world," he said. Some locals listened to him from the balconies of private apartments, a glass of champagne in their hand.

- Bells peal -

Bells pealed across the principality to mark Leo's arrival in the microstate nestled on the Mediterranean between France and Italy.

Some 5,000 locals gathered outside the palace to hear the pope speak, many brandishing flags in the red and white of the principality and the yellow and white of the Vatican.

"I'm trembling - it's so emotional, and I'm so proud," said Alix Pearce, a 34-year-old sales representative who came with her family.

After his speech, more than 1,500 young people welcomed Leo to the square in front of the Church of Saint Devota, dedicated to the patron saint of Monaco.

But while cheers went up along the popemobile's route, the full-scale jamboree hoped for by the authorities did not pan out. 

In the tiny city state's streets, billboards showing the pontiff in his religious garb starkly contrasted with the gleaming sports cars and crowds of tourists.

"The pope brings people together," said Eric Battaglia, a 64-year-old Monegasque artist and musician.

"In a world that has been at war for the past few years, it's a blessing that there are people like him, trying to ensure that people remain human."

- 'Powerful sign' -

Prince Albert said in an interview with local daily Nice-Matin: "This visit is a powerful sign testifying to the principality's importance within the Catholic Christian world." 

The prince added he shared with the Vatican common causes including international solidarity and "the promotion of peace through sport".

Though some locals professed indifference to the papal visit, Isabel Fissore, the 62-year-old owner of a small jewellery shop, was enthusiastic.

"It's historic: the two smallest states in the world coming together to spread a message of peace, light and love throughout the world," said Fissore, who managed to get invites to meetings with the pontiff.

"We may be a small nation, yet our hearts are big."

At an open-air Mass later at the Louis II stadium, Pope Leo's address reaffirmed the Church's opposition to euthanasia and abortion, positions entrenched in Monaco's laws.

And in an allusion to the international situation, he condemned the "the hidden schemes of powerful authorities who are eager to kill without scruples", without specifying to whom he was referring.

"Let us not grow accustomed to the clamour of weapons and images of war," he added.

"It was magnificent," said 60-year-old art Belgian dealer Wendy Lauwers afterwards, as the pope left Monaco by helicopter.

"We had heard a lot about how if he was coming here it was to talk to the rich. But he was very approachable, kind and open," she said.

"He told us 'Okay, you are privileged -- but stay humble'."