It was Gore's campaign to force global warming on to the international political agenda that inspired the event.
He invited the crowd at the city's Aussie stadium, to take Live Earth's seven-point pledge to reduce their personal environmental impact and support policies to reduce greenhouse gas emissions.
Gore, standing before the Capital building in Washington DC, said: "Thank you for coming today and thank you for being the very first to launch this movement to help solve the climate crisis. Enjoy the show."
The relatively unknown funk, Blue King Brown took the stage shortly afterward, the first official band on the global bill.
The worldwide line up includes Madonna, Metallica, the Police and Kanye West.
Problems and changes to the series continued right down to the last minute.
A ninth concert, in the US capital, was only added on Friday, while the Rio event was nearly stopped by a judge who feared for the safety of the 700,000 expected to attend the free concert in Copacabana.
Critics say Live Earth lacks achievable goals, and that bringing in jet-setting rock stars in fuel-guzzling aeroplanes to plug in amplifier stacks and crank up the sound may send mixed messages about energy conservation.
Organisers say the concerts will be as green as possible, with a record of energy use being kept.
They say proceeds from ticket sales will go towards distributing power-efficient light bulbs and other measures that will offset the shows' greenhouse gas emissions.
Rolling west through Saturday, the series starts in Sydney, then Tokyo, Shanghai, Johannesburg, Hamburg, London, Rio De Janeiro, New Jersey and Washington DC.
The biggest names will appear in London and New Jersey, with more modest line ups of mostly local and regional acts in the other cities.
Organisers were predicting live broadcasts on cable television and the internet could reach up to two billion people.
Scores of short films and public service announcements will be aired giving the audience tips about how to conserve energy and reduce their environmental impact.