Portugal fears a fourth wave of the coronavirus pandemic may take hold, with the highly contagious Delta variant accounting for more than 60 per cent of new cases in the capital.
Lisbon is among a dozen places that did not have the final lockdown phase removed.
Travel between the capital region and the rest of Portugal was banned from last weekend to halt the spread of infection.
First identified in India, the Delta variant has become the main strain in the greater Lisbon area, the national health institute Insa says.
"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, Ms Temido said, at a time when many European countries are easing 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 that carry heavy social and economic consequences," she said.
With the number of daily cases soaring by 54 per cent last week, Portugal found itself ahead of Britain with Europe's fastest growth rate for infections, an AFP tally showed.
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 said.
"It begins with a very slow phase of growth during which everything seems under control, then it explodes."
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", Prof Carmo Gomes said.
But the appearance of the Delta variant was a "nasty surprise" as the gradual easing of safety measures was well under way.
German Chancellor Angela Merkel on Tuesday criticised the EU's lack of travel rules after Portugal reopened its borders to European nationals and UK people in mid-May.
"The big question is, will the vaccinations still be delivered quickly enough to counter the spread of the infection," Prof Carmo Gomes said.
In official figures published on Tuesday 46 per cent of the 10 million population have received one dose of a vaccine and 29 per cent are fully vaccinated.
But the number of Covid cases in hospital has more than doubled in a month to 450 patients, including about 100 in intensive care.
Portugal's President Marcelo Rebelo de Sousa said on Monday that hospital figures only showed 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, Mr de Sousa 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," Mr Costa said.