Real estate experts in Paris have warned about the danger of people cramming on to balconies around the city to catch a glimpse of the Olympics in July and August this year.

The National Real Estate Federation (FNAIM) has alerted local authorities to the risk of collapses and accidents unless balconies that are often designed for two or three people are checked beforehand for their structural soundness.

The issue is set to be particularly pressing for residential blocs that overlook the river Seine, which athletes will sail down during a spectacular opening ceremony being planned by organisers on July 26.

"We need to be absolutely sure that the balconies can take the weight and that handrails are well sealed in to avoid any sort of incident," the head of FNAIM in the Paris region, Olivier Princivalle, told AFP.

Accidents involving balconies are a rare but sometimes deadly occurence in France, with two people left seriously injured in the southeast of Paris in May last year when their fifth-floor balcony gave way.

Four people died in the central city of Angers in 2016 when a balcony collapsed during a party.

The Paris Olympics from July 26-August 11, followed by the Paralympics from August 28-September 8, are set to take place at locations across the City of Light where many buildings are hundreds of years old.

The issue of balcony safety underlines the immense organisational complexity faced by local authorities as they prepare the first Games in Paris in 100 years.

The opening ceremony -- the first time a summer Olympics has begun outside of a stadium -- represents a huge challenge for French police who have been asked to secure a 6.0-kilometre (four-mile) stretch of the river that will be used during the parade.

The Paris police department told AFP that checking balconies did not fall under its responsibilities, but it was working with the industry and its partners to address the issue.

Under French law, building owners or managers have responsibility for checking their structures, but "something can slip through the cracks," a French source with knowledge of the preparations told AFP, on condition of anonymity.

Advance warnings about security and transport restrictions during the Games have led many Parisians to plan holidays during the event, sometimes in order to rent out their homes for high prices to foreign visitors.

"Don't leave this summer, don't leave! That would be a mistake," mayor Anne Hidalgo urged the city's inhabitants on Sunday as she inaugurated the only new permanent Olympics sports arena to be built in inner Paris.

"It's going to be incredible."