PARIS (AP) — As the countdown clock to the Paris Olympics passed the 100-day mark, the question of holding the opening ceremony on the Seine River loomed large over the milestone celebrations on Wednesday.

Organizers have planned a parade of about 10,500 athletes through the heart of the French capital on boats on the Seine along a 6-kilometer (3.7-mile) route in the opening ceremony at sunset on July 26.

But French President Emmanuel Macron said on Monday that the exceptional open-air event in central Paris could be shifted to a more conventional opening ceremony at the Stade de France if the security threat is deemed too high.

“If we think there are risks, depending on our analysis of the context, we have fallback scenarios. There are plan Bs and plan Cs,” Macron said.

France is on high security alert ahead of the Paris Olympics and Paralympics, which are expected to draw millions of visitors to the country. Organizers, French government officials and French athletes at the J-100 (“100 Days To Go” Olympic event) on Wednesday all acknowledged security concerns over the opening ceremony, which will bring huge crowds and over 100 world leaders to the embankments.