Pope Francis presided over the service, which was attended by some 50,000 mourners.
Benedict in 2013 became the first pope in 600 years to retire, and he chose to live out his retirement in seclusion.
Pope Francis is next month set to make what would be his first visit outside Rome since Italy was put under lockdown in early March when it became the first country in Europe to feel the full brunt of the coronavirus pandemic.
The Vatican is telling bishops around the world they should report case of clergy sex crimes to police even when not legally bound to do so.
Praying in a desolately empty St. Peter's Square, Pope Francis has likened the coronavirus pandemic to a storm laying bare people's illusions that they can be self-sufficient.
Pope Francis has canceled his planned day trip to Malta on May 31 due to the coronavirus.
Pope Francis has left the Vatican to make a surprise visit to two churches. The move Sunday came even as Italian health authorities demanded that people to stay home as much as possible.
Pope Francis is further limiting his public appearances to prevent crowds from gathering as a precaution against the spread of coronavirus.
Pope Francis has canceled his official audiences after apparently coming down with a cold.
Officials say Pope Francis is sick and he skipped a planned Mass with Rome clergy across town on Thursday.