Hundreds of thousands of people will be able to watch the opening ceremony of the Paris Olympics for free, organisers stressed Tuesday amid ongoing criticism about the price of tickets for next year's event.

The final figure for the number of people who will be granted tickets for the vast and ambitious outdoor opening ceremony along the river Seine is still under discussion.

"Hundreds of thousands," Interior Minister Gerald Darmanin told a press conference when asked about the number of people on Tuesday. "It will depend on the weather and the publicity you do for it."

Around 100,000 tickets will be sold for exclusive river-side positions, with organisers initially saying another 500,000 would be given an opportunity to watch from higher positions on roads above the Seine.

Athletes are set to sail down the river in a flotilla of 115 boats, the first time the opening ceremony for the Olympics has taken place outside of the athletics stadium.

"With its open and public nature, this ceremony will enable hundreds of thousands of people to see it for free," Paris mayor Anne Hidalgo told the same press conference with Darmanin and Games chief Tony Estanguet.

With demand far outstripping supply, Paris 2024 organisers have faced a stream of criticism online and even from some athletes over the price of tickets, which first went on sale in February and March.

A second release, which began on May 11, has led to outrage over prices as high as 2,700 euros ($2900) for the paying positions for the opening ceremony.

"I'm not even sure that my family will be able to come to see me, it's so expensive," Belgium's Nafissatou Thiam, a two-time Olympic heptathlon champion, told Belgian media DH recently.

The ceremony also poses a huge security challenge for French organisers, with around 10,000 athletes taking part and 200 heads of state set to attend in addition to the huge crowds on the river banks.

Around 37,000 members of the security forces would be on duty for the opening ceremony, Darmanin said, with summer holidays cancelled for police over the period of the Games which will run from July 26 to August 11.

Around 2,000-3,000 private security agents are also being recruited for the opening ceremony.

Darmanin stressed drones were considered the biggest security threat, but he added air defence technology was available and would be trialed later this year during the Rugby World Cup in France.

"It's a new threat. It's not certain that anything will happen but it is certainly the most difficult to prepare for," he said.