Biden vows to end 'season of darkness'

Former US vice-president Joe Biden said the first step he would take if he wins the White House would be to get control of the COVID-19 pandemic that has ruined hundreds of thousands lives in the United States, a country which he vowed to unite and lead out of a "season of darkness".

Biden, 77, made the remarks as he accepted the Democratic presidential nomination on Thursday night, setting up an election battle with Republican President Donald Trump under the grim shadow of an unprecedented health and economic crisis.

"The current president has cloaked America in darkness for much too long. Too much anger, too much fear, too much division," Biden said.

"If you entrust me with the presidency, I will draw on the best of us, not the worst," he declared. "And make no mistake, united we can and will overcome this season of darkness in America."

Biden, who previously sought the White House in 1988 and 2008, spoke from his hometown of Wilmington, Delaware, capping the fourth and last night of the virtual Democratic National Convention.

His 22-minute speech appeared to be eloquent, clear and sometimes emotional. It drew applause from supporters online and those at a drive-in watching party in Wilmington, as broadcast live on TV. The speech also dispelled criticism heaped on him by his rival Trump, who had tweeted doubts about his mental capacity and called him "Slow Joe" or mocked him as "Sleepy Joe".

On Thursday night, Trump continued to rip into Biden as his nomination acceptance speech went on.

"In 47 years, Joe did none of the things of which he now speaks," the president tweeted. "He will never change, just words!"

Mandatory mask wearing

Biden also slammed Trump, lamenting that the US is now leading the world in deaths, as the pandemic had claimed at least 173,100 lives in the US by Thursday night.

Biden vowed that on the first day of his presidency he would implement a national COVID-19 plan and mandate mask wearing, things that he said should have been done from the very beginning.

After the speech, Biden was shown holding hands with his wife Jill Biden, and walking outside with his running mate, Kamala Harris, the Democratic nominee for vice-president, and her husband, to greet their waiting supporters as fireworks lit the sky.

Cal Jillson, a political scientist and historian at Southern Methodist University in Dallas, said the virtual format of the convention was forced on the Democrats, and even more so on the Republicans, but Democrats have made it "tighter and more fast-moving" than the traditional national party conventions.

"Biden leads by a comfortable margin, seven or eight points in most polls, so the goal of the convention is to solidify that support and to encourage disaffected Republicans to stay open-minded during the general election campaign," Jillson said.

The Republican convention next week has the more difficult task of winning back moderate Republicans and cutting into Biden's lead, he said.

According to Jillson, an incumbent administration has to defend actual results, how it has handled the novel coronavirus, the economic decline and the social unrest, while the challengers can criticize and declare that they would have done better on all fronts.

"In difficult times, the challengers have the advantage," he said.

June Chang in San Francisco contributed to this story.