Former US First Lady Michelle Obama has capped the first day of a virtual Democratic National Convention, with a powerful declaration that Donald Trump is the “wrong president for our country”.

“He has had more than enough time to prove that he can do the job, but he is clearly in over his head,” she said.

“He cannot meet this moment. He simply cannot be who we need him to be for us. It is what it is.”

Wearing a necklace that spelled out “vote”, Ms Obama – the wife of former president Barack – urged Americans to elect Mr Biden to end the chaos created by Mr Trump’s presidency.

“We’ve got to vote for Joe Biden like our lives depend on it,” she said.

Watch Michelle Obama’s speech

Acknowledging she did not care much for politics, Ms Obama said Mr Biden’s steady and empathetic approach to problems was the answer and urged voters to stand in line or do whatever it took to ensure they could beat Mr Trump.

“Whenever we look to this White House for some leadership or consolation or any semblance of steadiness, what we get instead is chaos, division and a total and utter lack of empathy,” she said.

“If you take one thing from my words tonight, it is this: If you think things cannot possibly get worse, trust me, they can; and they will if we don’t make a change in this election.”

Her speech ended day one of the four-day convention to formally nominate Mr Biden as the Democratic challenger to Mr Trump in the November 3 US election.

Earlier, former presidential candidate Bernie Sanders, once Mr Biden’s hottest rival, spoke of needing “a movement, like never before”.

“In response to the unprecedented set of crises we face, we need an unprecedented response – a movement, like never before, of people who are prepared to stand up and fight for democracy and decency,” he said.

The coronavirus pandemic forced Democrats to overhaul the convention, largely eliminating the in-person gathering planned for Milwaukee, Wisconsin, and turning it into two-hour, prime-time packages of virtual speeches and events.

The convention opened amid widespread worries about the safety of voting in November because of the coronavirus pandemic.

Democrats have pushed mail-in ballots as a safe alternative, but fear it could be hindered by cost cuts at the Postal Service that, under Louis DeJoy, a top Trump donor, have led to delays in mail service.

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The convention featured discussions with voters from across the US who described their struggles confronting the virus and coping with the slumping economy and health care.

It also highlighted a call for a broad racial reckoning over systemic racism and police brutality amid protests that broke out after the death of African-American George Floyd in Minnesota beneath the knee of a white policeman.

Speaking from Houston, Mr Floyd’s brother Philonise introduced a moment of silence and honoured other black victims of police violence.

John Kasich, a former Ohio governor and frequent Trump critic who lost to Mr Trump in the 2016 Republican primary, described his opposition to the President as a patriotic duty.

“I’m a life-long Republican but that attachment holds second place to my responsibility to my country,” he told the convention.

Other Republicans to speak included former New Jersey governor Christine Todd Whitman and former Hewlett Packard Enterprise chief executive Meg Whitman.

Their inclusion angered some Democrats, who voiced concern it would take time away from progressive speakers such as Mr Sanders and US Representative Alexandria Ocasio-Cortez.

Mr Biden leads Mr Trump in national opinion polls heading into back-to-back convention weeks for the two political parties.

Mr Trump will be formally nominated for a second term at next week’s Republican National Convention.

Mr Biden, 77, who was vice-president under Mr Obama, will be formally nominated on Tuesday as the Democratic challenger to Mr Trump, 74, in the November 3 election. His acceptance speech will cap the convention.

Mr Biden’s vice-presidential pick, US senator Kamala Harris, will speak on Wednesday.

-with AAP