King Abdullah of Saudi Arabia has met Pope Benedict XVI at the Vatican - the first audience by the head of the Roman Catholic Church with a Saudi monarch.
The Vatican described the private meeting as "warm" and said the two men discussed the presence and hard work of Christians in Saudi Arabia.
An estimated 1.5m Christians live in Saudi Arabia but are not allowed to worship publicly.
The Vatican said Abdullah requested the audience as part of a European tour.
The two sides have no diplomatic ties, although when Abdullah was crown prince he met the late Pope John Paul II.
Correspondents say the visit comes as relations between the Vatican and the Muslim world are improving, more than a year after the crisis caused by a papal speech appearing to associate Islam with violence.
The 84-year-old Saudi monarch is on the third leg of his European tour after visiting the UK and Switzerland. He will travel next to Germany and Turkey.
Pope Benedict warmly greeted King Abdullah at the Vatican on Tuesday, grasping both his hands before leading him to a library for their brief private meeting, which lasted only 30 minutes, with both leaders speaking through interpreters.
Afterwards, the king offered his host a gold sword encrusted with jewels. He was given a 16th Century engraving of the Vatican in return.
The Vatican said the talks allowed a wide discussion on the need for religious and cultural dialogue among Christians, Muslims and Jews "for the promotion of peace, justice and spiritual and moral values, especially in support of the family."
Both sides also emphasised the need for a "just solution" to the Israeli-Palestinian conflict, the Vatican said.
About a million Catholics, many of them migrant workers from the Philippines, live in the conservative desert kingdom, which is the home of Islam's holiest shrines.
They are allowed to worship in private, mostly in people's homes, but worship in public places and outward signs of faith, such as crucifixes, are forbidden.
Christians complain that rules are not clear and hardline Muslim authorities sometimes crack down on legitimate congregations.
"The most important thing is to get the possibility to gather in freedom and security for our worship, our masses and our activities," said Bishop Paul Hinder, responsible for Catholics in Arabia, in an interview with Reuters news agency.
The Saudi authorities cite a tradition of the Prophet Muhammad that only Islam can be practised in the Arabian Peninsula.
King Abdullah, who is styled the Custodian of the Two Sacred Mosques - in Mecca and Medina - is an advocate of cautious reform in Saudi Arabia, often against the wishes of the powerful conservative religious establishment.
The BBC 's Frances Harrison in Rome says the symbolism of the meeting was huge for those who believe there should be more dialogue between Islam and Christianity, especially after the pontiff's controversial September 2006 speech at Regensburg University. In it, he quoted Emperor Manuel II Paleologos of the Byzantine Empire, who said in the 14th Century that the Prophet Muhammad had brought only "evil and inhuman" things.
The pope later stressed that these had not been his own words and expressed regret for any offence they had caused.