The archbishop of Paris and Catholics from around France and the world honoured the firefighters who saved Notre-Dame Cathedral.
Some streets around the medieval cathedral also reopened on Sunday, six days after the blaze, allowing tourists to get a closer look and local restaurants to reopen after firefighters declared the last hot points extinguished.
Notre-Dame itself is expected to remain closed for years.
The blaze that engulfed the cathedral on Monday night forced parish members and visitors who hoped to worship at Notre-Dame on Easter to find other places to attend services.
The Paris diocese invited them to join Sunday's mass at the grandiose Saint-Eustache Church on the Right Bank of the Seine River.
The firefighters, who struggled for nine hours to contain flames that consumed Notre-Dame's roof and collapsed its spire, held a place of honour at the Saint-Eustache.
Paris Archbishop Michel Aupetit handed firefighters at the Easter mass a book of scriptures that was rescued from Notre-Dame.
"Your men were able to save many things in the cathedral. But you also saved an item that is precious for us," Aupetit said. "It still is a bit dirty, full of ashes and likely a bit damaged by the fire. You saved it, and I wanted to give it to you. It's a very humble way to thank you," he said.
He notably thanked fire service chaplain Jean-Marc Fournier, who recovered from the fire a chalice containing consecrated hosts that for Catholics are the body of Christ.
Speaking of Jesus' resurrection, Aupetit told the congregation, "We, too, brothers and sisters, we will rise again, just like our cathedral will rise again."
The French archbishop also shared thoughts for "our Sri Lanka brothers who were massacred" in Easter attacks on churches and hotels that killed more than 200 people and injured hundreds more.
Notre-Dame's parishioners were joined by Catholics and others from around France and beyond. An Associated Press reporter heard at least six languages being spoken in the crowd.
"Everyone is affected by what happened to Notre-Dame," Parisian Michel Ripoche said. "Easter is a holiday we celebrate every year, all our lives. Clearly what happened at Notre-Dame added to the importance" of today's service.
Construction workers strung netting across one of the cathedral's prized rose windows on Sunday, apparently to protect the centuries-old stained glass.
French Culture Minister Franck Riester said on Sunday that most of the spots at Notre-Dame that were considered vulnerable to more damage or collapse have been stabilised, including support structures above the rose windows. He told France-2 television "there remain some sensitive points in the vaulted ceiling".
Australian Associated Press