Notre Dame will hold its first Mass since a devastating fire broke out on the roof of the cathedral in April, the city's diocese said Tuesday. According to AFP. the Mass will be held on Saturday to mark the anniversary of the dedication of cathedral's altar, celebrated annually on June 16.

Archbishop of Paris Michel Aupetit will lead the small Mass, which will take place in a "side chapel with a restricted number of people, for obvious security reasons," the diocese said.

Only 20 people are expected to take part, but the Mass will be broadcast by a French television channel so other worshipers can watch live. Those in attendance are expected to wear hardhats for safety, Catholic New Agency reports.

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The fire that broke out on April 15 destroyed Notre Dame's iconic spire and reduced its ancient wooden roof to ash. However, photos from inside the cathedral showed its cross and altar apparently still intact.

French President Emmanuel Macron vowed to rebuild the landmark. In an address to the nation after the devastating blaze, Macron said he wanted it done within five years, but an expert told CBS News that reconstruction could take decades.

"It's going to be a case of assessing the damage, strengthening everything that's there, do a full inventory of what we've lost, and then find the building materials," Dr. Emily Guerry, Senior Lecturer in Medieval European History at Britain's University of Kent, told CBS News. "In the modern world, we don't build like we used to."

The reconstruction will likely take billions of dollars. Shortly after the fire, French billionaires and donors around the world had pledged more than $1 billion to rebuild the cathedral. But a spokesman for the cathedral said that still might not be enough to cover it.

"We don't know if it's enough or not," Andre Finot told CBS News' Roxana Saberi. "You can say it's crazy to have a billion euros but we don't know. Maybe the cost is two or three billion."