Hundreds of Harley-Davidson enthusiasts from all over the world received the pope's blessing on Sunday, drawing cheers and applause from pilgrims gathered in St Peter's Square for a pro-life mass.

Pope Francis, riding through the throngs aboard his popemobile, made the sign of the cross toward the bikers, who are in Rome as part of the European celebrations of Harley Davidson's 110th anniversary.

The pontiff, who accepted two Harley Davidsons and a jacket from the bikers on Wednesday, addressed them again during his weekly Angelus prayer, saying: "I greet the many participants in the Harley-Davidson motorcycle assembly."

Some 35,000 bikers had gathered in the Italian capital by Friday, enjoying rock concerts by artists such as Adam Ant and Bonnie Tyler at Ostia beach outside Rome as well as taking part in a parade.

The celebrations were dampened by an accident on Saturday involving two cars and six bikes that left eight bikers injured, one seriously.

In Sunday's mass in St Peter's Square, Francis said: "All too often, people do not choose life, they do not accept the 'Gospel of Life' but let themselves be led by ideologies and ways of thinking that block life, that do not respect life."

Instead, they "are dictated by selfishness, self-interest, profit, power and pleasure, and not by love, by concern for the good of others," he said.

"As a result, the Living God is replaced by fleeting human idols which offer the intoxication of a flash of freedom, but in the end bring new forms of slavery and death," the 76-year-old told the throng estimated by the Vatican to number some 100,000.

"Let us say 'Yes' to life and not death. Let us say 'Yes' to freedom and not enslavement to the many idols of our time," Francis said.

In May, the Argentine pontiff joined pro-life marchers at the March for Life in Italy and spoke out against abortion, calling for "respect for human life from the moment of conception."

The new pope, who was elected in March, called for "legal protection of the embryo, protecting human life from the very start."