US President Barack Obama has implored voters to help continue his legacy by carrying Hillary Clinton to victory at the polls in November.
Mr Obama praised Mrs Clinton at the Democratic convention in Philadelphia, as the most qualified person ever to run for the White House.
He said voters faced a choice between hope and fear, attacking "homegrown demagogue" Republican Donald Trump.
But Mr Trump rejected the president's optimistic portrayal.
"Our country does not feel 'great already' to the millions of wonderful people living in poverty, violence and despair," he said on Twitter.
Vice-President Joe Biden also delivered his own spirited address, saying Mr Trump "backs torture", "religious intolerance" and "betrays our values".
"He has no clue about what makes America great," Mr Biden said, before the crowd started chanting, "Not a clue!"
The president closed the evening by recounting his key victories over his past eight years in office and painting an optimistic future for the country.
Mr Obama extolled Mrs Clinton's character, calling her a "leader with real plans to break down barriers, blast through glass ceilings and widen the circle of opportunity to every single American".
However, he also addressed Mr Trump's grim portrait of America, noting it was "not the America I know".
Instead, Mr Obama described his nation as "full of courage", "decent and generous", but also concerned about racial divisions and "frustrated with political gridlock".
Praising the qualities of American people, Mr Obama addressed them directly when he said: "Time and again, you've picked me up. I hope, sometimes, I picked you up, too.
"Tonight, I ask you to do for Hillary Clinton what you did for me. I ask you to carry her the same way you carried me."
The former secretary of state joined him on stage for a hug after he finished speaking.
In his response, Mr Trump said in a statement that the Democrats had described a vision of America that did not exist for most people.
Seventy percent of Americans thought the country was on the wrong track, said the hotel developer, who defied all predictions to win the Republican primary contest.
"Never has a party been so disconnected from what is happening in our world."
'Not one word'
Virginia Senator Tim Kaine took the stage to "humbly" accept his party's nomination for vice-president and then go on the attack.
"Folks, you cannot believe one word that comes out of Donald Trump's mouth," he said.
The crowd then erupted into chants of "Not one word!"
In other highlights from Wednesday night:
Former Arizona congresswoman Gabby Giffords, critically injured in a 2011 shooting, described Mrs Clinton as a president who would "stand up to the gun lobby"
Former CIA director Leon Panetta was heckled with chants of "no more war!" as he tried to warn of dangerous foreign policy under a Trump presidency
Ex-New York Mayor Michael Bloomberg skewered fellow billionaire Donald Trump, saying: "The richest thing about him is his hypocrisy"
Christine Leinonen, mother of one of the victims of the Orlando nightclub shooting, made an emotional plea for gun control
Hillary Clinton will officially accept the nomination on Thursday, setting up an election fight with Mr Trump that will take them to the presidential election in November.