The former US President Barack Obama has severely criticised the response to the Covid-19 pandemic ravaging America, in a rare public rebuke of his successor Donald Trump.

His remarks came as he spoke at two virtual commencements for high school and university students locked down instead of celebrating their graduations, a rite of passage every year in the United States.

"More than anything, this pandemic has fully, finally torn back the curtain on the idea that so many of the folks in charge know what they're doing," Mr Obama told graduates from several dozen historically black colleges and universities at the first ceremony.

"A lot of them aren't even pretending to be in charge," he said as he discussed the implications of the pandemic during the online event, without naming any specific leaders.

The celebration was followed by a second, star-studded prime-time ceremony for high school students in which the former president continued along the same lines, telling students to step up where leaders had failed.

He said: "If the world is going to get better, it's going to be up to you."

"Doing what feels good, what's convenient, what's easy - that's how little kids think.

"Unfortunately, a lot of so-called grown-ups, including some with fancy titles and important jobs, still think that way, which is why things are so screwed up.

"I hope that instead, you decide to ground yourself in values that last, like honesty, hard work, responsibility, fairness, generosity, respect for others."

The ceremony, carried by all the major American television networks, flashed between stirring speeches and performances students and celebrities alike, including basketball superstar LeBron James and singers the Jonas Brothers.

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Mr James echoed Mr Obama's call for the students to step up, urging them to "stay close to home" and help rebuild communities devastated by the virus and shutdowns.

The US leads the world with coronavirus cases and deaths, at 1.4 million and nearly 90,000, respectively.

This year's US graduates face a grim reality: the unemployment rate has shot up to nearly 15% and more than 36 million people have filed for unemployment benefits as America grapples with the fallout from months of lockdown and business closings.

The gravity of Mr Obama's message was underscored by the fact it was the second time in a week he had hit out at the response to the pandemic, having pointedly refused to criticise President Trump in public in the three years since leaving office.

He kept his profile low despite being frequently lambasted by Mr Trump.

But on 9 May, in remarks leaked from a web call with people who worked in his administration, he called Mr Trump's handling of the pandemic a catastrophe.

In his comments yesterday, Mr Obama also highlighted how the health crisis has underscored racial inequality in America, and expressed outrage at the shooting dead of 25-year-old Ahmaud Arbery, killed on 23 February while out for a jog in Georgia.

"A disease like this just spotlights the underlying inequalities and extra burdens that black communities have historically had to deal with in this country," he said at the first ceremony.

"We see it in the disproportionate impact of Covid-19 on our communities.

Without naming Mr Arbery, he continued: "Just as we see it when a black man goes for a jog and some folks feel like they can stop and question, and shoot him, if he doesn't submit to their question."