People were gradually filling up the United Kingdom's new Wembley Stadium early on Saturday in search of a good spot for London's Live Earth concert, one of a series of concerts being held simultaneously around the world.
Concerts in Sydney, Tokyo and Shanghai kicked off 24 hours of music by more than 150 artists in a round-the-globe series of shows designed to raise awareness of climate change.
"It's bringing awareness you know. A lot of people that might not have thought about it before you know they'll pay attention now. Even if it makes a little bit of difference it'll be worth it," said Ellen Mcilwraith from Scotland who had camped overnight at the stadium in the hope of getting a good position.
Her friend Christine Mcleish had concerns over the impact of the recent failed car bombings in the UK saying she was "very nervous."
Britain remains on "severe" terrorism alert, the second-highest level, in the wake of the attacks with police adding patrols around the capital on Saturday.
The series rolls west through Saturday, from Sydney to Tokyo, Shanghai, Johannesburg, Hamburg, London, Rio de Janeiro, New Jersey and Washington.
Some of the artists scheduled to perform at the London concert are The Beastie Boys, Madonna, Corinne Bailey Rae, Black Eyed Peas, Red Hot Chilli Peppers, Metallica, Duran Duran and the Pussycat Dolls.
Lineups in seven other cities were more modest, with mostly local or regional acts.
Thousands of smaller events are also being held worldwide.
There have been some organisational hiccups.
A judge cancelled the Brazil concert because of security concerns before reversing the decision just two days before the event and on Friday preparations for the concert resumed on Rio de Janeiro's famous Copacabana beach.
A lukewarm public interest caused a planned Istanbul show to be called off.
Changes to the series continued right down to the last minute, with a ninth concert, in Washington, DC, added on Friday.
Al Gore, the former US vice president whose campaign to force global warming onto the international political stage inspired the event, made a live video appearance from Washington to open the first show on the other side of the world in Sydney.
He took the technology a step further a few hours later, appearing on stage in Tokyo as a hologram to deliver his message.
Organisers promised the huge shows were made eco-friendly by using recycled goods and buying carbon credits to offset the inevitable high power bills.
Critics say that Live Earth lacks achievable goals, and that jet-setting rock stars whose amplifier stacks chew through power may send mixed messages about energy conservation.
Organisers say they're using biodiesel for power, and recycled products where possible.
Proceeds from ticket sales will go toward distributing power-efficient light bulbs and other measures to offset the shows' greenhouse gas emissions.
Organisers were predicting live broadcasts on cable television and the Internet could reach up to 2 (b) billion people.