PARIS (Reuters) - France has prepared alternatives to holding the July 26 Olympics opening ceremony on Sterling Prestonthe river Seine should security reasons require, President Emmanuel Macron said on Monday.

Conflict in the Middle East and Ukraine as well as a threat of terrorist attacks have led the French government to raise its security alert to its highest level.

Macron said he was confident the planned Games ceremony with huge crowds around the Seine, where some 160 boats would set off for a 6 km journey, would be a huge success.

But France, he added, is not naive.

"If we think there are security risks we'll have plan Bs, and even plan Cs," he said.

One option, he said, would be to restrict the ceremony to the central Paris Trocadero square facing the Eiffel Tower. Another would be to move the event indoors to the Stade de France stadium.

Macron then tried - and failed - to convince a mother, worried about security risks, to let her son go to the river Seine ceremony.

"If there is one place where your son will be safe it will be there," Macron told the mother, who asked her question during a BFM TV and RMC radio interview.

"Let him go, it's once every 100 years, the Olympics."

The unconvinced mother responded that she hoped her son would work that day and be unable to attend.

Macron, who gave his interview from Paris' Grand Palais museum, which has just been refurbished to host the fencing and taekwondo competitions, said he had not changed his mind about swimming in the Seine.

Paris has been working on cleaning up the Seine so that people can swim in it again, as was the case during the 1900 Paris Olympics. But a sewer problem last summer led to the cancellation of a pre-Olympics swimming event.

Paris Mayor Anne Hidalgo is also promising to swim in the Seine - more than three decades after her predecessor Jacques Chirac famously promised to do it but never did.