VATICAN CITY (Reuters) - Pope Francis on Sunday proclaimed as saints some 800 Italians killed in the 15th century for refusing to convert to Islam, and said many Christians were still being persecuted for their faith.

The Vatican seemed at pains not to allow the first canonizations of Francis' two-month-old papacy to be interpreted as anti-Islamic, saying the deaths of the 'Otranto Marytrs' must be understood in their historical context.

The 800 were killed in 1480 in the siege of Otranto, on the southeastern Adriatic, by Ottoman Turks who sacked the city, killed its archbishop and told the citizens to surrender and convert.

When they refused, the Ottoman commanding officer ordered the execution of all men aged 15 or older, most by beheading.

"While we venerate the Otranto Martyrs, we ask God to sustain the many Christians who, today, in many parts of the world, right now, still suffer violence and give them the courage to be faithful and to respond to evil with good," Francis said before more than 70,000 people in St. Peter's Square.

He did not mention any countries, but the Vatican has expressed deep concern recently about the fate of Christians in parts of the Middle East, including Coptic Christians who have been caught up in sectarian strife in Egypt.

A booklet handed out to participants said the "sacrifice" of the Otranto Martyrs "must be placed within the historical context of the wars that determined relations between Europe and the Ottoman Empire for a long period of time".

HEALING MIRACLE

Francis' predecessor, Pope Emeritus Benedict XVI, made a speech in Regensburg, Germany, in 2006 that was perceived by Muslims as equating Islam with violence.

Benedict said at the time he had been misunderstood. He is now living in retirement in a Vatican convent, after stepping down in February.

Roman Catholic sainthood requires that two miracles be attributed to those who are being made saints - one before beatification, and another before canonisation.

In the case of the 800 Italians, the requirement for the first miracle was waived because they were killed "in hatred of the faith".

The miracle approved for their canonization was that of a nun who had cancer which, according to the Church, was healed after she prayed at a memorial to the martyrs in Otranto.

At Sunday's ceremony, Francis also proclaimed Colombia's first saint, a nun who died in 1949, and canonised a Mexican nun who died in 1963.

At the end of the Mass, he made his first appeal as pope against abortion, saying life must be "respected from the moment of conception" and throwing his support behind an Italian group promoting legal protection for embryos.

(Editing by Mark Trevelyan)