Francis also rejected claims by some conservatives that Benedict, who died on Aug. 31, was embittered by some of the current pope's decisions.
More than 1,000 Italian security personnel were called up to help safeguard the event, and air space around the tiny Holy See has been closed off for the day.
The retired former pope has been laying in state without the papal regalia, but will be buried in the traditional three coffins in the Vatican.