 Duran Duran performed four songs at Wembley |
Concerts in London, Johannesburg and Hamburg are under way, while events in Sydney, Tokyo and Shanghai have ended.
Among acts who have already played at London's Wembley Stadium are the Red Hot Chili Peppers and Genesis.
The day has been organised by former US Vice-President Al Gore, as part of his campaign to try to "heal the planet".
Mr Gore appeared at a smaller event at the National Museum of the American Indian in Washington, giving a speech which was relayed to the concerts around the world, calling on the developed world to reduce carbon emissions by 90%.
Concerts in New Jersey and Rio get under way later.
Organisers say the gigs will reach an audience of two billion people via TV, radio and online broadcasts.
But critics have said it was hypocritical for performers who fly around the world on tours to push the message of cutting down on carbon emissions.
George Marshall of the Climate Outreach Information Network told the BBC: "Having the richest people in the world saying, 'Hey! We all need to cut back a bit!' is, let's face it, absurd."
Speaking from Wembley, Snow Patrol lead singer Gary Lightbody told BBC Radio 1: "We're here to learn how to make our tours cleaner. We already offset our travel on our touring, but our shows themselves are quite far behind.
"We're not the Red Hot Chili Peppers, who've been offsetting for years - and more power to them. It's a band like that and an event like this that you come here to learn [from]."
Reformed group Genesis, featuring Phil Collins, was among the first acts to perform at London's Wembley Stadium, starting their set with Turn it On Again.
Aboriginal welcome
The Red Hot Chili Peppers, James Blunt and the Beastie Boys are due on stage later.
Duran Duran, the only band to have also performed at last weekend's memorial Diana concert, opened their set with the perfect song for the occasion, Planet Earth.
"Everyone who did not arrive on a private jet put your hands in the air," said lead singer Simon Le Bon, who also raised his hand.
German concert-goers will be treated to performances by Snoop Dogg, Enrique Iglesias and Yusaf Islam, while UB40 and Joss Stone are performing in Johannesburg.
The Sydney event began with a traditional aboriginal welcome before Australian politician and former Midnight Oil frontman Peter Garrett made an appearance, saying it was up to citizens of developed nations to push for action to reduce pollution.
"Your voice matters, make it heard," he said.
Reformed New Zealand group Crowded House were joined by many of the other Sydney performers at the end of their set, for a rendition of the 1991 hit Weather with You.
Lead singer Neil Finn told fans they were "the groundswell" for action on the environment.
'Heal the planet'
The Tokyo event was opened by the band Genki Rockets at the Makuhari Messe hall, east of the Japanese capital.
Among the performers in Tokyo was US metal group Linkin Park.
"I don't have any delusions of grandeur that Linkin Park is saving the world," bassist Phoenix told BBC News.
"But it's our goal to do what we can to make it possible for our kids to have a place to live, and, at the same time, hopefully clean up where we live in the process."
Japanese singer Ayaka urged people to do what they could. "We can start helping by doing something small," she said.
"I started to carry my own eco-bag so I don't have to use plastic grocery bags, and use my own chopsticks instead of disposable ones."
Responding to criticism that the event creates even more carbon emissions, organisers have insisted they were keeping the concerts as green as possible, with proceeds being spent on power-efficient light bulbs and other measures to offset the shows' emissions.
"We've booked this show with acts that were touring in the area at the time so we could keep the carbon imprint down," explained producer Kevin Wall.