Pope Francis met Hungary's anti-migration Prime Minister Viktor Orban on a whistlestop trip to the country on Sunday in which he called for greater openness, before heading for a four-day tour of Slovakia.

During a Hungarian visit that lasted just seven hours, the pope had urged tens of thousands thronging the vast Heroes' Square in Budapest that he wanted them to be "grounded and open, rooted and considerate".

The pope will celebrate an open-air mass on the Hungarian capital's vast Heroes' Square The pope will celebrate an open-air mass on the Hungarian capital's vast Heroes' Square Photo: AFP / ATTILA KISBENEDEK

The head of 1.3 billion Catholics has often urged help for the marginalised and those of all religions fleeing war and poverty, in contrast to Orban who styles himself as a defender of "Christian Europe" from immigrants.

Thousands more crowded a nearby main boulevard, along which screens and loudspeakers had been set up, while others watched from nearby balconies and other buildings.

"The pope never says anything without reason. His words are well chosen and carry a subtle message," said Zsuzsanna Pusztai, 75.

Rounding off his stay the pope will celebrate an open-air mass on Heroes square in Budapest Rounding off his stay the pope will celebrate an open-air mass on Heroes square in Budapest Photo: AFP / ATTILA KISBENEDEK

The pope, in Hungary to close the International Eucharistic Congress, met Orban and other senior politicians.

"I asked Pope Francis not to let Christian Hungary perish," Orban posted on his Facebook page, along with a photo of the two shaking hands.

Over the last few years, there has been no love lost between supporters of Hungary’s Prime Minister Viktor Orban and the leader of the Catholic world Over the last few years, there has been no love lost between supporters of Hungary’s Prime Minister Viktor Orban and the leader of the Catholic world Photo: AFP / Jure Makovec

As a present, he gave the pontiff a copy of a letter written by King Bela IV to Pope Innocent IV in 1250 asking for help against Mongol warriors who threatened Christian Hungary.

Hungarian Prime Minister Viktor Orban's signature crusade against migration has included border fences and detention camps for asylum-seekers Hungarian Prime Minister Viktor Orban's signature crusade against migration has included border fences and detention camps for asylum-seekers Photo: AFP / ISTVAN HUSZTI

The Vatican described the meeting as "cordial", saying they discussed environmental protection and the promotion of the family among other topics.

There has been no love lost between Orban supporters and the Catholic leader.

Rounding off his stay the pope will celebrate an open-air mass on Heroes square in Budapest Rounding off his stay the pope will celebrate an open-air mass on Heroes square in Budapest Photo: AFP / ATTILA KISBENEDEK

Pro-Orban media and political figures have called Francis "anti-Christian" for his pro-refugee sentiments.

Earlier, Francis told Hungarian bishops that various ethnic and religious groups had "transformed this country into a multicultural environment", presenting a "great opportunity".

In contrast, Orban's signature crusade against migration has included border fences and detention camps for asylum-seekers.

Days before the pope's arrival posters appeared on the streets of the Hungarian capital -- controlled by the opposition -- reading "Budapest welcomes the Holy Father" and showing his quotes including pleas for solidarity and tolerance towards minorities.

Meeting Christian and Jewish leaders earlier in the day, Francis warned of "the threat of antisemitism still lurking in Europe and elsewhere".

During that speech the 84-year-old remained seated, apologising that "I'm not 15".

Hungary's 100,000-strong Jewish community is one the largest in Central Europe.

Orban has regularly been accused of running anti-Semitic campaigns with posters depicting Hungary-born financier George Soros as a puppet master controlling the EU's migration policy.

On his arrival in the Slovak capital, Bratislava, Francis addressed representatives from Christian denominations.

"It is hard to expect Europe to be increasingly influenced and enriched by the Gospel if we... are not yet fully united and are unconcerned for one another," he said.