Democratic vice presidential nominee Joe Biden has made an unscheduled appearance at Invesco Field, touting the "open convention" at Denver's football stadium.

His assignment was to precede a lineup of Democrats from many walks of life, speakers given a place on the convention's climactic night to help set up Barack Obama's speech accepting the presidential nomination.

Biden told the delegates: "We're here for the millions of Americans who have been knocked down, to send a message together to send a message that we're going to get back up as a nation."