Portugal fears a fourth wave of the coronavirus pandemic may take hold with the highly contagious Delta variant now accounting for more than 60 percent of new cases in the capital.
Lisbon is among a dozen places that did not move into the final phase of easing the lockdown that much of the country has enjoyed.
Travel between the capital region and the rest of Portugal was banned from last weekend to try to halt the spread of the infection.
First identified in India, the Delta variant has become the predominant strain in the greater Lisbon area, according to the national health institute INSA.
"We are trying to delay its arrival in other regions of the country so that people can protect themselves more through vaccination," Health Minister Marta Temido said Monday.
More restrictions may be necessary, she added, at a time when many European countries are easing such curbs for summer.
"We have to assess it as we go along and we are asking for everyone's support, to avoid as much as possible measures which carry heavy social and economic consequences."
With the number of daily cases soaring 54 percent last week, Portugal found itself ahead of Britain with Europe's fastest growth rate for infections, according to an AFP tally of data from national authorities.
Over seven days, the daily average of new infections has topped 1,100 cases, compared with 300 six weeks ago.
"We have seen exponential growth since the month of May," Lisbon University epidemiology professor Manuel Carmo Gomes told AFP.
"It begins with a very slow phase of growth during which everything seems under control, then it explodes," he said.
With strict confinement measures imposed from mid-January to mid-March, "we have shown that it is possible to control the epidemic without keeping people at home", the professor said.
But the appearance of the Delta variant came as a "nasty surprise" with the gradual easing of safety measures well under way.
German Chancellor Angela Merkel criticised on Tuesday the European Union's lack of bloc-wide travel rules after Portugal reopened its borders to EU national and Britons in mid-May.
"No going back?"
"The big question is, will the vaccinations still be delivered quickly enough to counter the spread of the infection," said Carmo Gomes.
In official figures published Tuesday 46 percent of the 10 million population have received one dose of a vaccine and 29 percent are fully vaccinated.
However the number of Covid cases in hospital has more than doubled in a month to 450 patients, including about 100 in intensive care.
President Marcelo Rebelo de Sousa said Monday that current hospitalisations were only a third of the red line figure representing an overload on the health system.
The nation is "very far" from the situation that required a health emergency to be declared in the six months leading to May, he said.
The conservative head of state, who has no executive power, had declared there would be no going back to a lockdown.
But socialist Prime Minister Antonio Costa took a cautious position. "No one can guarantee that we will not return to a lockdown," he said.