In the wake of the Christchurch attack, the Kiwi leader highlighted that tech companies could do a lot more in tackling the proliferation of violent content online.
Mattarella and Macron, who was accompanied by his wife Brigitte, began their visit at the royal chateau in Amboise, where the heads of state laid wreaths at Leonardo's grave.
Notre Dame tragedy: 1,100 experts call on French President Emmanuel Macron for time to rebuild the cathedral
France's government last week presented a bill aimed at speeding up the reconstruction of Notre Dame that would allow workers to skip some ordinary renovation procedures.
Macron said that the dramatic fire had brought out the best in a country that has been riven with divisions.
Sanders called Notre Dame, which was heavily damaged in the blaze, an "iconic structure" that "has served as a spiritual home for almost a millennium."
Experts said that it may take decades for the damaged structure to be set up.
French President Emmanuel Macron told reporters near the scene that he will seek international help, including from the "greatest talents" in the world, to rebuild Notre Dame.
UK MPs have rejected the withdrawal agreement May reached with other European leaders in 2018, so she is now asking for the leaving date to be extended.
Macron has repeatedly been accused of harbouring an arrogant, dismissive attitude towards ordinary people who complain about having difficulties making ends meet.
Macron is under pressure to avoid a repeat of last week's sacking of the Champs-Elysees, where over 100 shops were damaged, looted or set alight during seven hours of rioting.
Wednesday's meeting comes one day after British Prime Minister Theresa May said Parliament would get the chance to vote to delay Britain's scheduled March 29 departure from the European Union.
The crowds marched in rain from the Place de la Nation square to the Bastille monument, some chanting "Yes to democracy, no to revolution" as they waved French and European Union flags.
Marches were underway in several other cities across France.
The letter, which said that Macron "failed to understand the legitimate expectations of the French people," was published on Thursday by the Facebook group called "La France en colere" (Angry France).
Visiting a tax query call centre in the northern city of Amiens, Budget Minister Gerald Darmanin attempted to assure the French that the change would be painless.