Pope Francis has presided over the marriage of 20 couples, in the first papal wedding ceremony at the Vatican in 14 years.

Among the couples were several who were cohabiting and one couple who had children.

The Vatican views sex outside marriage as sin, but Pope Francis has stressed that the Church should be a forgiving one.

In another signal of the openness of his papacy, Pope Francis asked to marry 40 people from different social backgrounds who would be a realistic sample of modern couples.

Marriage "is not an easy road, it's sometimes a contentious trip, but that's life", the pontiff told the couples.

"It's normal that couples fight. That always happens, but don't end the day without making peace, even a small gesture is enough."

The ceremony on Sunday was the first of its kind at the Vatican since Pope John Paul II presided over a wedding in 2000.

One of the couples he married was single mother Gabriella and her partner Guido, whose previous marriage was annulled by an ecclesiastical tribunal.

The ceremony at St Peter's Basilica in Rome comes three weeks before a meeting of Catholic bishops from around the world to discuss the issues of marriage, divorce and contraception.

Pope Francis has said the Church must end its obsession with teachings on abortion, contraception and homosexuality, and become more merciful, or risk collapsing "like a house of cards".

His approach contrasts with that of his predecessor, the German Pope Benedict, who said threats to the traditional family undermined the future of humanity itself.

Last January, Pope Francis baptised the child of a single mother in the Sistine Chapel along with the daughter of a couple who had not been married in a church.

Reuters/AFP