A Polish couple and their seven children, killed by Nazis during World War II for hiding Jews, were beatified Sunday, the first time an entire family was given one of Catholicism's highest honours.

Papal envoy Cardinal Marcello Semeraro led the ceremony in the family's hometown of Markowa in south-east Poland, which was attended by thousands including the president and prime minister, bishops, priests, the country's chief rabbi and an Israeli delegation.

"May this Polish family, who represented a ray of light in the darkness of the Second World War, be for us all a model to imitate in the impulse of goodness, in the service of those in need," Pope Francis said on Sunday.

It was in Markowa, on March 24, 1944, that German police acting on a tip-off shot dead Jozef Ulma and his wife Wiktoria, who was seven months pregnant and partially gave birth during the execution.

Their children, Stanislawa, Barbara, Wladyslav, Franciszek, Antoni and Maria, aged between two and eight, were killed too, along with the eight Jews the family had been hiding in the attic.

The eight -- Shaul Goldmann and his five children, including his daughter Lea Didner and her five-year-old daughter, and Golda Gruenfeld -- were also shot, before the family farmhouse was looted and set on fire.

The police fired up into the attic from the floor below, "and the blood of the victims began to drip from the ceiling... onto a photograph of two Jewish woman lying on a table below", Vatican News said.

That photograph "has been preserved as a 'relic'", it said.

- Baptism of blood -

The massacre followed "a story of love and friendship", said Italian journalist Manuela Tulli, who has written a book on the family along with Polish historian and priest Pawel Rytel-Andrianik.

"When the Jews asked for help, they opened their doors. They lived together for a year and a half, cooking and eating together", Tulli told AFP.

Jozef Ulma was a keen photographer as well as a farmer, and photographs he took that survive reveal the family's life through simple, everyday scenes.

"We see the children running barefoot in the grass, doing their homework, the mother hanging out the washing," Tulli said.

The families were denounced by a Polish policeman. And after they were executed, 24 Jews in Markowa were murdered by their Polish neighbours.

The Ulma family are the first ever to be beatified, a key step on a possible path to sainthood in the Catholic Church.

And in a rare move, the Ulmas' newborn seventh child also earned the title of "blessed".

The child was eligible for beatification through the concept of "baptism of blood", having been born "at the time of the mother's martyrdom", according to the Vatican's department for saints.

Usually people need to have performed a miracle to be eligible for beatification, but martyrs are exempt.

Jozef and Wiktoria Ulma were recognised by Israel in 1995 as members of the "Righteous among the Nations", an honour for non-Jews who tried to save Jews from Nazi extermination.

The family also has a museum dedicated to it in Markowa and in 2018 Poland decreed 24 March -- the date of the massacre -- a day of remembrance for Poles who rescued Jews during the German occupation.