PARIS (AP) — Skies over the Paris region will be closed for six hours as part of the massive security operation for the July 26 opening ceremony of the Olympic Games, the Paris airports operator said Wednesday.

Augustin de Romanet, chairman of Aéroports de Paris, said airlines are being warned in advance about the closure and told they will have to fly around the restricted airspace.

“For six hours, there won’t be any aircraft over the Paris region,” he said on France Info radio.

The no-fly zone will extend for a radius of 150 kilometers (93 miles) around Paris, the civil aviation authority and Interior Minister Gerald Darmanin have said.

The unprecedented waterborne ceremony on the River Seine running through the French capital is the stiffest single security challenge for Paris Games organizers, with crowds of more than 320,000 people expected to line the waterway.