Pope Francis has married 20 couples in St Peter's Basilica - with one bride already a mother.

Pope Francis officiated a mass at the wedding of 20 couples in St Peter's Basilica at the Vatican. Photo: Alberto Pizzoli, Reuters

Pope Francis in his homily today likened families to the "bricks that build society".

Among the couples, all from the Rome area, who said 'I do' - or 'Si - 'was one in which the groom's first marriage was annulled by the church and the bride has a daughter she had while an unmarried woman in an earlier relationship.

The Vatican views sex outside marriage as a sin, but the Pope stresses the church should be a forgiving one.

He said marriage was "real life, not some TV show". He told couples to expect conflicts but that Jesus' love would help them when love runs out. Church teaching opposes divorce.