Covid-19 infections have risen by 10% in a week in Europe after two months in decline and the risk of a new wave of cases is growing, says the World Health Organization.
Regional director Hans Kluge said the risk had been heightened by sluggish vaccine rollouts, new variants and increased social mixing.
There was also a danger that Euro 2020 could act as a "super-spreader".
Hundreds of fans returning from London and St Petersburg have tested positive.
WHO senior emergency officer Catherine Smallwood called on host cities to do more to monitor the movement of fans.
"What we need to look at is around the stadia," she said, highlighting travel before and after matches. "What's happening after the games? Are they going into crowded bars and pubs?"
Rise of Delta variant
The Delta variant, which originated in India, is seen as the biggest threat by many countries in Europe. The European Union's disease control agency ECDC estimates that it could account for 90% of cases by the end of August.
Russia has seen record numbers of deaths for the past three days, with 672 fatalities and 23,543 new cases announced on Thursday alone. Most of the new cases in Moscow are of the Delta variant, and top health officials are also talking of a new Delta-plus variant.
Euro 2020 host city St Petersburg recorded 115 deaths in the past 24 hours on Thursday, on the eve of its sixth and last tournament match between Spain and Switzerland.
Finnish health authorities appealed to the public to avoid travelling to Russia after 400 infections were linked to fans returning from St Petersburg on 21 June.
European football's governing body, Uefa, was branded "irresponsible" by German Interior Minister Horst Seehofer, who said it was clear that hugging supporters would help spread the virus. He was particularly critical of the decision to allow 60,000 supporters into the stadium at Budapest and at Wembley in London for the semi-finals and final.
"I cannot explain why UEFA is not being sensible... I suspect it is due to commercialism," he told reporters. Uefa is adamant that decisions on the number of fans allowed inside a stadium "fall under the responsibility of the competent local authorities".
Public Health Scotland said on Wednesday that 1,294 Covid cases had been linked to people who had travelled to London for the Euro 2020 match against England on 18 June, including 397 fans who were at Wembley.
Across the UK, another 27,989 cases have been recorded, the highest number since January. However, 62.7% of UK adults have had two vaccine doses.
In other developments:
- Portugal is imposing a night-time curfew on Lisbon, Porto and other municipalities from Friday because of the biggest increase in cases since February
- Spain has reported a sharp rise in its incidence rate among people in their twenties - up to 366 cases per 100,000 people
- Germany expects the Delta variant to account for over 70% of cases this month so may relax quarantine measures on visitors from the UK and Portugal where the variant is already prevalent.
The EU's medicine agency says that two doses of the vaccines it has so far approved appear to provide protection against the Delta variant.
While vaccine rollouts have accelerated across the EU in recent months and at least one in three people have had both doses, that is not the case in Russia.
President Vladimir Putin appealed on Wednesday for Russians to get vaccinated, however only 16% have had a single shot so far. Moscow health clinics have begun offering booster jabs in an attempt to curb the spread, however local reports say supplies of at least one vaccine have run out in the city's clinics.
Hans Kluge said that across the WHO's Europe region, which includes Central Asia, only 24% of people were covered and half of elderly people and 40% of health care workers remained unprotected.