Face masks became compulsory in tourist hotspots in Paris on Monday amid warnings of a resurgence of coronavirus cases, as infections in the United States topped five million.
The world is expected to surge past two grim landmarks in the coming days: 20 million coronavirus cases and 750,000 deaths.
But World Health Organisation chief Tedros Adhanom Ghebreyesus insisted there is still hope of conquering the pandemic.
"Behind these statistics is a great deal of pain and suffering... But I want to be clear: there are green shoots of hope," he said Monday, adding that "it's never too late to turn the outbreak around".
He gave examples of countries that had successfully clamped down on COVID-19, such as Rwanda and New Zealand, which said Monday it plans to open a virus-free "travel bubble" with the Cook Islands.
With much of the world caught in a cycle of dispiriting outbreaks and economically crushing lockdowns, all eyes are on the race for a vaccine.
A WHO overview said 165 candidate vaccines are being worked on around the world, with six reaching Phase 3 of clinical evaluation.
But WHO's emergencies director Michael Ryan warned that a vaccine was "only part of the answer," pointing to polio and measles as diseases with vaccines that have not been fully eradicated.
"You've got to be able to deliver that vaccine to a population that want and demand to have that vaccine," he said.
Europe feels the heat
Infections have been rising ominously in Western Europe, which has also been sweltering through a heatwave, with temperatures soaring above 35 degrees Celsius (95 F).
The blistering heat sent crowds flocking to beaches at the weekend despite health warnings about the risk of infection.
In the Paris region, people aged 11 and over are now required to wear masks in crowded areas and tourists hotspots.
These include the banks of the Seine River and more than a 100 streets in the French capital.
Marion, a 24-year-old in central Paris, said the masks are "restrictive" but necessary "if we want to avoid a second wave".
"Anything except a second lockdown," she added.
Several French towns and cities have already introduced similar measures, as well as parts of Belgium, the Netherlands, Romania and Spain.
As cases surge in his homeland, Spanish actor Antonio Banderas announced on Monday -- his 60th birthday -- that he has tested positive for coronavirus.
The movie star said he was feeling "relatively well" and would use him his time in quarantine "to read, write, rest and continue making plans to start giving meaning to my recently completed 60 years".
In Berlin, thousands of children returned to school on Monday after the summer break, sporting masks which are compulsory in common areas like school courtyards.
Greece meanwhile announced a night curfew for restaurants and bars in some of its top tourist destinations after its number of new cases increased.
In Italy, the coronavirus spikes of its neighbours caused alarm.
"France, Spain and the Balkans... Italy is surrounded by contagions," Italian Health Minister Roberto Speranza lamented.
It was a different story in Pakistan, which allowed all restaurants and parks to reopen on Monday, after the country saw a drop in new cases over several weeks.
Grim US, Brazil milestones
The world's worst-hit country the United States has recorded nearly 163,000 coronavirus deaths, and on Sunday passed the extraordinary milestone of five million cases.
President Donald Trump's Democratic opponent in the presidential election, Joe Biden, tweeted that the number "boggles the mind and breaks the heart."
The figure came as Trump was accused of flouting the constitution by unilaterally extending a virus relief package.
The package -- announced by Trump on Saturday after talks between Republican and Democrat lawmakers hit a wall -- was "absurdly unconstitutional," senior Democrat Nancy Pelosi told CNN.
But with the world's largest economy still struggling to dig itself out of an enormous hole, Democrats appeared skittish about any legal challenge to a relief package they see as seriously inadequate.
After the US, Brazil has the most cases, and over the weekend it became the second country to pass 100,000 fatalities.
President Jair Bolsonaro has played down the coronavirus threat, and after Brazil's latest milestone the country's most widely viewed TV network Globo asked: "Has the president of the republic done his duty?"
In Peru, indigenous people armed with spears and angry over what they consider government neglect of their communities in the pandemic assaulted a settlement for oil workers deep in the Amazon, triggering a clash with police that left three dead, the government said.
In Spain, top-flight football club Atletico Madrid reported on the weekend that two players -- Angel Correa and Sime Vrsaljko -- tested positive to coronavirus.
But further tests on players and staff came back negative, allowing the team to travel to Portugal to face Germany's Leipzig on Thursday for a place in the Champions League semi-finals.