Although the extent of the damage is not estimated yet, donations of about 700 million euros (790 million U.S. dollars) have been pledged to rebuild the 850-year-old Gothic building Notre Dame Cathedral.
Two of France's most rich businessmen, Francois-Henri Pinault, chief executive of the Kering group which owns brands including Gucci and Yves Saint Laurent, and Bernard Arnault, the main shareholder of luxury group LVMH, opened the donation campaign. They offered 100 million euros and 200 million euros of donations respectively.
That was followed by 100 million euros pledged by energy giant Total and 50 million euros by Paris city hall. French luxury and cosmetics group L'Oreal also will donate 200 million euros while other companies pledged donations ranging between 10 and 20 million euros.
Local media reported that the fire, which broke out at around 1645 GMT, may be linked to renovation works.
The inquiry to determine how the fire broke, with the involvement of 50 investigators, would be "long and complex", warned Paris prosecutor Remy Heitz.
He said the blaze was probably caused by accident as no signs "at this stage" pointed to a criminal purpose.
The cathedral, whose construction began in 1160 and continued over a century, is part of the World heritage site of "Paris, Banks of the Seine" inscribed on the World Heritage List in 1991. It receives 13 million visitors yearly. (1 euro = 1.128 U.S. dollar)