Pope Benedict XVI left the Vatican on Thursday for the last time as leader of 1.2 billion Roman Catholics, his helicopter cheered by onlookers in St. Peter's Square.

The Pope, who official was to end his papacy at 8 p.m. (Central European Time) flew to Castel Gandolfo, the papal summer palace south of Rome that overlooks a lake.

Unresolved issues such as transparency of the Vatican and an inability to deal fully with sex scandals of prelates dating back decades that were covered up, dogged the pontificate of Benedict who is the first pope in 600 years to step down voluntarily.

Earlier Benedict bade farewell to 144 cardinals saying, "Among you, among the College of Cardinals, there is also the future Pope, to whom, here today, I already promise my unconditional reverence and obedience."

Benedict said, "As I said yesterday to the thousands of the faithful who filled St Peter's Square, your closeness and your advice has been of great help to me in my ministry.

"In these eight years we have lived with faith the most beautiful moments of radiant light on the Church's journey, alongside moments when several clouds had gathered in the sky. We sought to serve Christ and his Church with profound and total love, which is the heart and soul of our ministry."

The Pope spoke to his closest advisers in the Vatican's Clementine Hall for his final greeting before leaving the papacy vacant and allowing a secret conclave that will be convened to elect a new pontiff.

The president of the Lutheran World Federation, Bishop Munib Younan and the LWF general secretary, Rev. Martin Junge sent a joint letter to Pope Benedict expressing gratitude for his contribution to the goal of Christian unity.

"We felt encouraged by the way you have been promoting the value of profound and honest theological discernment in ecumenical dialogues, because the quest for visible unity of the church belongs also to the foundational vocations of the Lutheran World Federation," they said.

They said they cherished the memory of meeting Benedict in December 2010 and the way the Pope underscored the shared vocation of Lutherans and Catholics, while urging reflection on their ongoing journey towards unity.

"With your leadership the Roman Catholic Church has strengthened the discussion on new evangelization and challenged also other Christian churches to reflect on their participation in God's mission in today's world," they added.