The audience at the Vatican on Saturday included Christians, Muslims and Jews.
Francis said strong inter-religious relations could help "alleviate the wounds of these dramatic events".
A relative shows Pope Francis a picture of a Nice lorry victim.
Nice's former mayor brought a basket of 86 flowers to symbolise the 86 victims of the July 14 attack by an extremist who mowed down people celebrating Bastille Day in the French Riviera city.
Francis cautioned against "responding to hatred with hatred and to violence with violence", although he said the temptation to do so is great.
Some at the meeting showed him photos of dead family members as he greeted participants one by one.
