Democratic presidential nominee Joe Biden has criticised US President Donald Trump for downplaying the severity of coronavirus, saying there is "a lot to be concerned about".
He said the president should be communicating the "right lesson" on masks and social distancing.
It came after Mr Trump told Americans not to fear Covid-19.
He has returned to the White House to continue his treatment for coronavirus after a three-night hospital stay.
The president, who is still contagious, removed his mask on the balcony of the White House while posing for pictures.
While he is no longer in hospital, his doctor has said he "may not entirely be out of the woods yet".
Speaking at an NBC television town hall event in Miami, Florida, on Monday night, Mr Biden said he was "glad" that the president seemed to be recovering well.
But, he said, "I would hope that the president, having gone through what he went through... would communicate the right lesson to the American people. Masks matter."
"The only thing I heard was one of the tweets saying that, you know, don't be so concerned about all this, essentially.
"There's a lot to be concerned about. Two hundred and ten thousand people have died," he added.
The US has recorded more coronavirus cases and deaths than any other country in the world.
Former First Lady Michelle Obama also criticised the president's response to the virus in a video released on Tuesday in which she urged people to vote for Mr Biden "like your lives depend on it".
"Seven months later, he [Mr Trump] still doesn't have a plan for this virus. Seven months later, he still won't wear a mask consistently and encourage others to do the same - even when those simple actions could save countless lives. Instead, he continues to gaslight the American people by acting like this pandemic is not a real threat," she said.
Mr Trump's diagnosis has upended his campaign for a second term in office, less than a month before the 3 November presidential election.
What has Trump done since his return?
Wearing a navy business suit, tie and mask, Mr Trump walked out of Walter Reed National Military Medical Center in the Washington DC suburbs on Monday evening pumping his fist.
After a short helicopter ride, Mr Trump was pictured alone on the Truman Balcony of the White House. He removed his protective face mask, before giving a thumbs-up and a military-style salute.
A couple of hours later, he tweeted a campaign-style clip of his return set to stirring music.
Mr Trump also recorded a video message, urging Americans to get back to work.
"You're going to beat it [coronavirus]," he told them, adding: "We're going to be out front. As your leader, I had to do that. I knew there's danger to it, but I had to do it. I stood out front, and led."
Mr Trump also speculated: "Now I'm better, maybe I'm immune, I don't know".
The World Health Organization says it is too early to know if people who have recovered from Covid-19 are protected from a second infection, and if so, how long this protection might last. The president's own medical team does not consider him to be fully recovered yet.
Mr Trump also promised that vaccines were "coming momentarily", although the head of the US Centers for Disease Control said last month that he did not expect a vaccine to be generally available before the middle of next year.
Before leaving hospital, the president told Americans in a tweet not to fear the disease and said he would be back on the campaign trail "soon".
According to US public health guidelines, people with symptomatic Covid-19 should remain in isolation for at least 10 days after symptoms first appear. The White House says the president first started to appear ill on Thursday evening, and later tested positive.
Mr Trump continued to post about coronavirus on social media on Tuesday, declaring on Twitter that he was "FEELING GREAT!"
Facebook deleted one post in which he claimed that Covid-19 was "less lethal" than the flu, while Twitter hid the same message behind a warning about "spreading misleading and potentially harmful information".
An almost messianic message
Donald Trump says he has overcome the coronavirus - and you can, too.
In his video message from the White House, a mask-less Trump tells the American public: "Don't be afraid of it. You're going to beat it."
And so the president's message in the final weeks of his re-election campaign takes shape. He contracted the coronavirus because he was an out-front leader and he "had to do that".
"Nobody that's a leader would not do what I did," he said.
It is a message almost messianic in its undertones - one that the rest of his party is amplifying. The president has suffered and overcome, and will lead the nation to a promised land beyond the virus.
New York Post columnist Miranda Devine, quote-tweeted by the president, said Mr Trump would return to the campaign trail as an "invincible hero". Georgia Senator Kelly Loeffler tweeted out a video of Mr Trump tackling a virus-headed antagonist.
There is political and personal risk for the president, of course. He could experience a relapse or long-term medical difficulties. Americans who have lost loved ones to the disease may find his words and actions ill-considered or offensive.
The president, however, seems determined to turn his recent weakness into a strength.
What did Trump's doctors say?
His physician Sean Conley said on Monday afternoon that the president, whose oxygen levels had dipped on Friday and Saturday, would be "surrounded by world-class medical care 24/7" at the White House.
He refused to answer questions about when Mr Trump last received a negative test or to go into the specifics of his treatment. He would not offer details regarding the president's scans to check for pneumonia, citing patient protection laws.
He did confirm Mr Trump is still on the steroid dexamethasone and has received three doses of antiviral drug remdesivir. He was due to receive another before discharge and a fifth at the White House.
When asked about whether Mr Trump was safe to travel for campaign events, Dr Conley said: "We'll see."
"If we can get through to [next] Monday with him remaining the same or improving, better yet, then we will all take that final deep sigh of relief," he added.
What the doctors will be looking out for
Considering we're told President Trump tested positive on Thursday, he seems to be making great progress - especially as he is a high risk patient, being 74, a man, overweight and living with a heart condition. He's received a cocktail of drugs normally reserved for the sickest of Covid patients who have been ill for weeks, not days, to aid and hasten his recovery.
It's not clear when Mr Trump caught the virus or first started to feel unwell. Most people who develop symptoms do so on around day five. Some people have only mild symptoms that clear up after a week or so with some bed rest.
A turning point can be around seven days later, with some patients getting a more serious lung infection from the virus. These patients need to be in hospital because they can deteriorate quickly and need expert care. The body can go into overdrive fighting the virus, leading to respiratory failure, septic shock and multiple organ damage.
Mr Trump's doctors say they remain cautiously optimistic about the president's health but "on guard" for at least the coming week should things take a turn for the worse.
What's the latest with the White House outbreak?
At least 12 people close to Mr Trump have now tested positive, as have several junior staff members.
First Lady Melania Trump, senior aides and several Republican senators are among those known to have coronavirus.
Mrs Trump, who is 50, has been isolating at the White House, reportedly with mild symptoms. In a tweet on Monday she said: "I am feeling good [and] will continue to rest at home".
White House Press Secretary Kayleigh McEnany became the latest high-profile figure close to the president to confirm a positive test on Monday.
Many of those attended a gathering at the White House Rose Garden on 26 September that is being scrutinised as a possible "super-spreader event".
The latest coronavirus case to emerge from that event, at which the president unveiled his nominee for the US Supreme Court, is a Christian minister from California.
Pastor Greg Laurie, who is said to have mild symptoms, was also with US Vice-President Mike Pence earlier that day at a prayer march in central Washington DC.
Plexiglass will be used to separate Vice-President Mike Pence and Democratic challenger Kamala Harris and limit the risk of Covid transmission when they go head-to-head in a debate in Salt Lake city, Utah, on Wednesday. Both candidates have recently tested negative.