The Louvre's ageing security camera system failed to detect thieves in time to prevent their audacious heist, the museum's director says as the museum reopened to visitors.

On Sunday, the thieves broke into the world-famous Paris museum using a crane to smash an upstairs window, then stole French crown jewels worth an estimated 88 million euros ($A157 million) before escaping on motorbikes.

"Despite our efforts, despite our hard work every day, we were defeated," Laurence des Cars, the Louvre director, told a Senate committee.

"We did not detect the thieves' arrival early enough," she said, blaming it on the fact that there were not enough cameras outside monitoring the vicinity of the museum.

The outside security cameras do not offer full coverage of the museum's facade, she said, add

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