With sunset and moonlight gleaming on the river, the grandiose opening ceremony for the Paris Olympics will last nearly four hours.

A total of 205 delegations will parade on more than 80 boats on the Seine. The ceremony will drift slowly from east to west, bridge to bridge, snaking 3.7 miles from Pont d'Austerlitz to Pont d'Iéna.

Proceedings begin at 3:45 p.m. and finish at 11:15 p.m. local time with artistic performances preceding the athletes' parade.

The athletes will motor past about 320,000 fans wedged in behind security cordons on upper and lower tiers on the embankments. Others will gaze at giant screens beaming images of the ambitious ceremony.

"For one evening, the Seine will be transformed into a giant open-air ceremony," Paris Games director of ceremonies Marie-Catherine Ettori said during a media briefing on Thursday.

Once the 10,500 athletes have disembarked from their boats, the final part of the ceremony takes place at the Trocadéro plaza overlooking the Eiffel Tower.

Amid the anticipation of seeing something so unique, there are tensions surrounding the safety of the ceremony heading into the Games starting on July 26.

Late last month, France raised its security readiness to the highest level after a deadly attack at a Russian concert hall and the Islamic State's claim of responsibility.

French President Emmanuel Macron struck a cautious note this week when he said the unprecedented open-air event, which is expected to bring 100 world leaders to the embankments, could be shifted to a more conventional opening ceremony at the Stade de France, the national stadium, if the security threat is deemed too high.

Without disputing what Macron said, France's Sports Minister Amélie Oudéa-Castéra and others remained optimistic the show will go on the Seine.

"We are heavily working on plan A which remains the central scenario and the very, very dominant scenario," Oudéa-Castéra said at a " 100 Days To Go " Olympic event on Wednesday. "We keep working on that fantastic ceremony on the River Seine."