Barack Obama told fellow Democrats in Chicago Tuesday that "the torch has been passed" to Kamala Harris and that the United States was ready for her to become president.

Former president Obama, who was greeted with rapturous applause and cheers at the packed arena hosting the party's nominating convention, said Vice President Harris would fight for Americans, and called her November poll rival Donald Trump "dangerous."

"Kamala Harris is ready for the job. This is a person who has spent her life fighting for people who need a voice," he said.

Obama called Harris "someone who sees you and hears you and will get up every single day and fight for you."

"Yes she can," Obama said of Harris, prompting the boisterous crowd to repeatedly chant the phrase, recalling Obama's own "Yes we can" campaign slogan.

Before his stardust performance, his wife and former US first lady Michelle Obama told convention goers "something magically wonderful is in the air."

"It's the contagious power of hope," she said, calling Harris "my girl" and saying that hope - another rallying cry of her husband's successful 2008 campaign - "is making a comeback."

His turn amped up the already buoyant mood in Chicago where President Joe Biden delivered his own emotional speech late Monday less than a month after ending his reelection bid.

"In 2012 I got to vote for him, and everyone was pushing Michelle Obama to run for president, but now we have Kamala. So I just think that this is, in a sense, them passing on the torch," said attendee Tomara Hall, 35, from California.

In deeply personal remarks shifting the focus onto Harris's qualities, her husband, Second Gentleman Doug Emhoff, told the convention "she is ready."

"She brings both joy and toughness to this task," he said to cheers.

"At this moment in our nation's history, she is exactly the right president."

With the party united and Harris polling strongly, Democrats are making clear they believe they can defeat Trump.

The Republican nominee had seemed set to regain power in November's election until Biden upended the race by dropping out and endorsing his vice president.

Comparisons are already being made by Democratic faithful to Obama's historic 2008 campaign, where a tidal wave of enthusiasm carried him to the White House.

Bullish delegates symbolically nominated Harris as their candidate in a boisterous roll call, following a paper exercise to confirm her as their standard bearer earlier this month.

"Thank you... see you in two days, Chicago," she said to delegates via video link from her event in Milwaukee.

Harris, who was received rapturously in Chicago at her debut appearance before Biden spoke, was in Milwaukee Tuesday for an event at the basketball arena where Trump attended the Republican convention just a month ago.

The choice of the 18,000-seat arena will rile Trump, who has been rattled that 59-year-old Harris, unlike Biden, is able to draw the kinds of crowds the Republican has long attracted to his events.

Addressing both crowds simultaneously highlighted that she had filled the DNC and RNC venues.

A mural by artist 4eene Vision depicting US minister and activist Martin Luther King Jr. (L), former US President Barack Obama (C) and US Vice President and 2024 Democratic presidential candidate Kamala Harris (R) in Los Angeles, California. Photo: AFP

'Hardest glass ceiling'

Trying to pry media attention away from the Democratic convention, Trump is holding events all week and on Tuesday spoke about what he says is Harris's "anti-police" stance.

At an event in Howell, Michigan, he attacked what he called "the Kamala crime wave."

"You can't walk across the street to get a loaf of bread -- you get shot," he said flanked by police officers and their cars, falsely claiming there has been a 43 percent increase in violent crime.

While allies have pleaded publicly for Trump to focus on policies and stop his barrage of personal insults against Harris, he has not stopped.

On Monday the DNC floor belonged to Biden, who delivered a swan song after being forced to abandon his reelection bid amid deep concerns that at 81 he is too old and frail to defeat Trump.

Biden has recast what might have been a humiliating moment into a narrative of sacrifice, passing on the torch to his younger protege.

"It's been the honor of my lifetime to serve as your president. I love the job, but I love my country more," he said, wiping away a tear amid thunderous applause before embracing Harris on stage.

Obama called Biden an "outstanding president" who had "defended democracy at a moment of great danger."

The other star speaker Monday was Hillary Clinton, who in 2016 was the first female presidential nominee of a major party, but lost to Trump in an election that opened up one of the most turbulent eras in recent US politics.

Harris, Clinton said, will be the one to break "the highest, hardest glass ceiling" in the country.

Twenty million people watched the DNC's first night, ratings monitor Nielsen said, beating viewers for the opener of the Republican gathering that drew 18.1 million.