Brazil has exceeded 2,000 Covid-related deaths in a single day for the first time, as infection rates soar.
The country has the second highest death toll in the world, behind the US, with a total of 268,370 fatalities. Wednesday's total was 2,286, reports BBC.
Experts warn the transmission rate is made worse by more contagious variants.
On Wednesday, former leader Luiz Inacio Lula da Silva hit out at President Jair Bolsonaro's "stupid" decisions made during the pandemic.
Mr Bolsonaro, who was pictured wearing a mask for the first time in a month on Wednesday, has consistently sought to downplay the threat posed by the virus. Earlier this week he told people to "stop whining".
The surge in cases has put health systems in most of Brazil's largest cities under immense pressure, with many close to collapse, Brazil's leading public health centre Fiocruz warns.
Margareth Dalcolmo, a doctor and researcher at Fiocruz said the country was "at the worst moment of the pandemic".
"2021 is still going to be a very hard year," she told AFP news agency.
What's the situation in Brazil?
On Wednesday, the country recorded 79,876 new cases, the third highest number in a single day. A surge in cases in recent days has been attributed to the spread of a highly contagious variant of the virus - named P1 - which is thought to have originated in the Amazon city of Manaus.
A total of 2,286 people died with the virus on Wednesday.
According to Fiocruz, 15 state capitals have intensive care units (ICUs) that are at more than 90% capacity including Rio de Janeiro and São Paulo.
Reports say the capital Brasilia has now reached full ICU capacity, while two cities - Porto Alegre and Campo Grande - have exceeded capacity.
In its report, the institute warned that figures point to the "overload and even collapse of health systems".
Brazilian epidemiologist Dr Pedro Hallal told the BBC's Outside Source TV programme: "If we do not start vaccinating the population here very soon, it will become a massive tragedy."
Dr Hallal, who works in Rio Grande do Sul, said people felt "abandoned by the federal government".
Mr Bolsonaro has belittled the risks posed by the virus from the start of the pandemic. He has also opposed quarantine measures taken at a regional level, arguing that the damage to the economy would be worse than the effects of the virus itself.
Former leader Lula, in his first speech since corruption convictions against him were annulled, told people not to follow "stupid" decisions by Mr Bolsonaro and to "get vaccinated".
Speaking on Wednesday he said "a lot of deaths could have been avoided".
Mr Bolsonaro said the criticism was unjustified.
What do we know about the Brazil variant?
Preliminary data suggests the P1 variant could be up to twice as transmittable as the original version of the virus.
It also suggests that the new variant could evade immunity built up by having had the original version of Covid. The chance of reinfection is put at between 25% and 60%.
Last week, the Fiocruz Institute said P1 was just one of several "variants of concern" that have become dominant in six of eight states studied by the Rio-based organisation.
"This information is an atomic bomb," said Roberto Kraenkel, of the Covid-19 Brazil Observatory, told the Washington Post newspaper.
World Health Organization Director General Tedros Adhanom Ghebreyesus has described the situation in Brazil as "very concerning" and warned of a possible regional spillover.
Which vaccines work against the variant?
Brazil has ordered more than 200 million doses of the Oxford-AstraZeneca and CoronaVac vaccine. So far, more than eight million people have had their first dose, representing just 4% of the population.
Preliminary studies of the AstraZeneca vaccine suggest it would protect against the P1 variant, although the team behind the vaccine earlier said it offers less protection - but should still protect against severe illness.
Another Brazilian study has reportedly indicated the CoronaVac vaccine, made by China's Sinovac biopharmaceutical company, is effective against the same variant.
The Pfizer-BioNTech vaccine has also been able to combat the Brazil variant, according to laboratory research published by the New England Journal of Medicine.
However, the Pfizer vaccine has not yet been rolled out in the country, as Brazilian authorities are still in negotiations with the company over its purchase. According to Reuters news agency, Economy Minister Paulo Guedes said Pfizer had agreed to deliver 14m doses by June after a video call with President Bolsonaro.