Pope Francis on Thursday opened a landmark summit at the Vatican on fighting child sex abuse, saying that the world expected "concrete measures" on tackling paedophilia in the Catholic church.

"The Holy people of God are watching and waiting not for simple and obvious condemnations but concrete and efficient measures," he said in a short opening statement.

Pope also prayed for help turning "this evil into a chance for understanding and purification."

He told the gathering of 190 leaders of bishops' conferences and religious orders that they bore the responsibility to "listen to the cry of the young who want justice."

Francis called the summit after botching a well-known sex abuse cover-up case in Chile last year. Realizing he had erred, he has vowed to chart a new course and is bringing the rest of the church leadership along with him.

The four-day meeting at the Vatican is to feature speeches, tutorials and workshops on protecting children, preventing abuse and stopping cover-ups.

More than 30 years after the scandal first erupted in Ireland and Australia and 20 years after it hit the U.S., bishops and other Catholic superiors in many parts of Europe, Latin America, Africa and Asia either deny that clergy sex abuse exists or downplay the problem.