Dion, a Canadian native, performed Edith Piaf's "Hymne à L'Amour" in French in front of a packed audience.
"France matters, Russia matters, Syria matters ... the entire world matters and peace is possible," Leto said. "Many of us here are the sons and daughters of immigrants."
Dion, 47, came out after Leto's speech and sang in front of a montage of Paris' most iconic landmarks. Dion sang in the way only the Grammy winner can, her voiced filling the auditorium as members of the audience had tears rolling down their faces. Also shown on the screen behind Dion were images of the French people uniting after the attacks.
The song ended with an image of the Eiffel Tower and the crowd rose for a standing ovation.
"We felt it was important to show our solidarity in light of the recent events in Paris and all around the world," producer Larry Klein said in a statement to Billboard before the show. "Celine's performance will help us express our feelings through song, when words do not suffice.”
The song was written by Piaf in the late 1940s after her lover, French boxer, Marcel Cerdan, died in a plane crash.
The Associated Press also contributed to this report.