French paramedic Seifelislam Benadda had just dropped off a patient at hospital on July 1, he said, when police informed him he was prohibited from leaving his hometown in the Paris suburbs, saying he was a potential threat to the Olympic Games.

For the next nine days, instead of driving his ambulance, the 28-year-old checked in at the Nogent-sur-Marne police station at midday and fought to overturn the administrative measure, which alleged he posed a terrorist risk.

As part of a vast security operation for the Paris Games, which start on Friday, authorities have turned to powers passed under a 2017 anti-terror law, placing 155 people under surveillance measures that strictly limit their movement and oblige them to register daily with police even though some have never faced criminal charges, according to official data and a Reuters review of cases.

France, which has a recent history of terror attacks, is on its highest state of alert ahead of the Games, including at Friday's opening ceremony on the Seine. In May, police arrested an 18-year-old suspected of planning a jihadist attack on the Saint-Etienne stadium, hosting Olympic football. Last week, a neo-Nazi was arrested on suspicion of plotting an attack during the passage of the Olympic flame.

Known as MICAS, the surveillance measures had until recently mainly been used to monitor people after prison sentences. In the context of the Olympics, Interior Minister Gerald Darmanin said at a news conference on July 17, the powers were only used to target people he described as "very dangerous" and potentially able to carry out attacks.