Pope Francis raised slain Salvadoran archbishop Oscar Romero and Pope Paul VI to sainthood on Sunday in a ceremony at the Vatican attended by tens of thousands of pilgrims.

"We declare and define Paul VI and Oscar Arnulfo Romero Galdamez... to be saints and we enrol them among the saints, decreeing that they are to be venerated as such by the whole church," Francis said, before the crowds broke into wild applause.

The pair were elevated to the highest position in the Roman Catholic Church along with five others, including an Italian orphan who died of bone cancer aged 19 and a German nun.

Relics of each new saint were carried to the altar -- part of a bone for Romero and the shirt Paul VI was wearing when he was stabbed in an assassination attempt at Manila airport in 1970.

Among those present were El Salvadoran president Salvador Sanchez Ceren, Chile's president Sebastian Pinera and Spain's Queen Sofia.

Francis was wearing a blood-stained rope belt which belonged to Romero, who stood up for peasant rights in the face of a right-wing backlash and was murdered at the altar in 1980.

The pontiff was also to use a chalice and pastoral staff belonging to Paul VI in a canonisation being seen as a reminder of Francis's call for "a poor church for the poor".

Both men have been hailed by Francis for their courage in turbulent times and their dedication to social justice and the downtrodden.

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