VATICAN CITY — Pope Benedict XVI promised his "unconditional reverence and obedience" to his successor in his final words to cardinals Thursday, a poignant and powerful farewell delivered hours before he becomes the first pope in 600 years to resign.

In an unexpected speech inside the Vatican's frescoed Clementine Hall, the pontiff appeared to be trying to defuse concerns about possible conflicts arising from the peculiar situation of having both a reigning pope and a retired one.

He also gave a final set of instructions to the "princes" of the church who will elect his successor, urging them to be united as they huddle to choose the 266th leader of the world's 1.2 billion Catholics.