Construction teams brought in a huge crane and a delivery of planks of wood to the site on Wednesday morning after French president Emmanuel Macron set a five-year deadline to restore the 12th-century landmark.

Contributors include tech giant Apple and magnates who own L'Oreal, Chanel and Dior, as well as ordinary Catholics and others from around the world.

Authorities have said the cause of the fire was accidental, possibly related to renovation work.

Firefighters are still examining damage and shoring up the structure after the fire collapsed the cathedral's spire and destroyed the roof.

Bells will toll at cathedrals around France on Wednesday evening in honour of the monument.

Remarkably, no one was killed in the fire, after firefighters and church officials evacuated the site quickly during a Mass.

The French government is gathering donations and setting up a special office to deal with larger offers.

Some criticism has already surfaced among those in France who say the money could be better spent elsewhere, on smaller struggling churches or helping workers.

Some 30 people have already been questioned in the investigation into the blaze, which the Paris prosecutor warned would be "long and complex".

Among those questioned are workers at the five construction companies involved in renovation work on the church spire and roof, which had been under way when the fire broke out.

A plan to safeguard the masterpieces and relics inside was quickly put into action once the alert was raised.

The Crown of Thorns, regarded as Notre-Dame's most sacred relic, was among the treasures quickly transported to safety, authorities said.

Brought to Paris by King Louis IX in the 13th century, it is purported to have been pressed onto Christ's head during the crucifixion.

The cathedral's famous 18th-century organ which boasts more than 8000 pipes also survived.

Some of the paintings and other artworks are being dehumidified, protected and eventually restored at the Louvre.

Australian Associated Press