PARIS — As the two women sat in deck chairs enjoying the last rays of sunshine near the Canal de l’Ourcq in Paris on Sunday evening, nearby loudspeakers jolted them with a reminder that they were in a new mask-mandatory zone.
“You’ve got your mask?” Safiya Zenag, unmasked, asked her friend, who replied: “No, I didn’t bring it. I hate wearing it.”
Faced with a recent resurgence of coronavirus cases, officials have made mask wearing mandatory in widening areas of Paris and other cities across the country, pleading with the French not to let down their guard and jeopardize the hard-won gains made against the virus during a two-month lockdown this spring.
The signs of a new wave of infection emerged over the summer as people began resuming much of their pre-coronavirus lives, traveling across France and socializing in cafes, restaurants and parks. Many, especially the young, have visibly relaxed their vigilance and have not followed rules on mask wearing or social distancing.
In recent days, France has recorded about 3,000 new infections every day, roughly double the figure at the beginning of the month, and the authorities are investigating an increasing number of clusters.
But 30 percent of the new infections are in young adults, ages 15 to 44, according to a recent report. Since they are less likely to develop serious forms of the illness, deaths and the number of patients in intensive care remain at a fraction of what they were at the height of the pandemic. Still, officials are not taking any chances.
“The indicators are bad, the signals are worrying and the situation is deteriorating,” Jérôme Salomon, the French health ministry director, told the radio station France Inter last week. “The fate of the epidemic is in our hands.”
Mr. Salomon warned that the virus would continue to circulate and that people would have to adjust their behavior. “We have to live with it,’’ he said.
France suffered 30,400 deaths from the virus — one of the world’s worst tolls — and experienced an economically devastating lockdown from mid-March to mid-May. Thanks to the lockdown, however, France succeeded in stopping the spread of the virus and lifted most restrictions at the start of summer.
Philippe Juvin, the head of the emergency department at the Georges Pompidou European Hospital in Paris, said he was not surprised by the rise in cases.
“You lock down people during two months, putting a stop to infections,” he said. “Once people are again allowed to go outside, it is not surprising that infection quickly resumes.”
The course of the pandemic in Europe has followed a somewhat similar trend, with Spain also reporting new local clusters. But important disparities exist among countries. In the past week, as France reported 20,000 new cases, Italy reported 7,000, and Britain, 3,000, according to data collected by The New York Times.
Mircea Sofonea, an epidemiologist at the French University of Montpellier, said today’s situation had “nothing to do in terms of imminent health risk” with the situation that preceded the European lockdowns because the number of hospitalized coronavirus patients and deaths remains very low.
In France, the daily number of deaths has hovered around 15 in the past week. By contrast, at the height of the epidemic in March and April, hundreds died every day in France, with the toll sometimes rising into four digits.
In April, intensive care units were at 140 percent capacity; only 7 percent were occupied about 10 days ago.
Mr. Sofonea said all European countries were expecting a rebound of the epidemic in the fall, when people who have been away on vacation come back to work and when social interaction resumes.
The French authorities fear that the rising number of infections in young people, many of whom are asymptomatic, may contribute to the spread of the virus to older, more vulnerable people.
“Young people felt a little more invincible,” said Olivier George, a 36-year-old baker. “That’s probably what made them the most affected group.”
Across the continent, crowds of young people are flocking to illegal parties organized in outdoor areas, regardless of the risk of infection.
While the number of new cases in France has been rising steadily, it is difficult to draw comparisons with earlier stages in the epidemic.
The number of tests being carried out across France has increased to about 600,000 a week — or about six times the numbers performed during the height of the epidemic. At that time, France suffered from severe shortages of test kits, making it impossible for many suspected of having Covid-19 to get tested.
Raphaëlle Escande, 23, a business school student, said she fell ill in March with symptoms of the disease, including the loss of smell, a sore throat and fever. “That lasted three weeks,’’ she said. “I stayed home because you couldn’t get tested.’’