Pope Francis told Europe's leaders on Friday the continent faced a "vacuum of values" as they marked the EU's 60th birthday, condemning anti-immigrant populism and extremism that he said posed a mortal threat to the bloc.

Prime ministers and presidents from 27 EU member states have descended on Italy to mark the 1957 founding Treaty of Rome, receiving a papal blessing on the eve of the anniversary.

However, celebrations have been tempered by a string of crises, including prolonged economic turmoil, an influx of migrants and Britain's decision to leave the bloc, that have raised fears for the future of the union.

"When a body loses its sense of direction and is no longer able to look ahead, it experiences a regression and, in the long run, risks dying," Francis told the leaders gathered in an ornate, frescoed chamber in the heart of the Vatican.