Pope Benedict XVI commanded his final set of instructions on Thursday to the cardinals of the church who will elect his successor, telling them to be together in unity as they elect the 266th leader of the world's 1.2 billion Catholics.

In his last hours as head of the Roman Catholic Church, he pledged to the "princes" that he would behave with "unconditional reverence and obedience" toward his successor.

Pope Benedict said he would pray for the cardinals in coming days as they discuss the issues facing the church, the qualities needed in a new pope and prepare to enter into the secret conclave to elect him.

"May the College of Cardinals work like an orchestra, where diversity - an expression of the universal church - always works toward a higher and harmonious agreement," he said.

After greeting more than 100 cardinals collectively from a throne in the Clementine Hall of the Apostolic Palace, he rose and thanked each of them. Dressed in a red and gold mantle lined with snow-white ermine, Pope Benedict held the cardinals' hands as they took off their red skullcaps to greet him and kiss his ring.

"I will be close to you in prayer," he said.

In his last address to St. Peter's Square on Wednesday, Pope Benedict said he wasn't returning to private life but would lean to a new form of service to the church through prayer.

At 8 p.m. sharp on Thursday Benedict became the first pontiff to resign since the Middle Ages. The palazzo doors will close and the Swiss Guards will go off duty, their service protecting the Pope will be over.

Benedict will initially live in Castel Gandolfo, a town outside Rome where other popes have stayed for centuries. He is expected to spend some time there before returning to live at the Vatican in a convent.