Congo has some of the world's richest mineral deposits, but its abundant resources have stoked conflict between ethnic groups, militias, government troops and foreign invaders.
He led the stadium in an impromptu chant of "no to corruption" in French, Congo's lingua franca.
"The pope is right," said Joel Muhemereri Amani, 21, an art student.
The United Nations says African economies lose nearly $150 billion to corruption each year.
The 86-year-old pope, who arrived in Congo on Tuesday, flies to neighbouring South Sudan on Friday.