Ed Sheeran, Lizzo and Billy Eilish are among stars taking part in a 24-hour concert to raise awareness of climate change, vaccine inequality and famine.
Over 60 artists are participating i in cities including New York, London and Seoul.
Tens of thousands of people are expected to attend, while millions are likely to tune in to the broadcast.
The event kicked off on Saturday in Paris, where Elton John performed in front of the Eiffel Tower.
He took to the stage following a pre-recorded performance by the K-pop band BTS.
The show, believed to be one of the biggest-ever international charity events, was organised by Global Citizen - a charity working to end extreme poverty by 2030.
Unlike most events of this nature, organisers are not aiming to raise cash but rather to use participation as evidence for world leaders that people support bold action on the issues.
"Across six continents, artists will help rally citizens in demanding that governments, major corporations and philanthropists work together to defend the planet and defeat poverty," Global Citizen said in a statement.
It called on international governments to plant one billion trees, deliver one billion vaccines to the poorest countries and donate meals for 41 million people on the brink of famine.
Jennifer Lopez was among those who performed in New York, where Billie Eilish and Coldplay are scheduled to play.
Earlier the Duke and Duchess of Sussex, Prince Harry and Meghan Markle, took to the stage calling for access to Covid-19 vaccines to be treated "as a basic human right."
"The way you're born should not dictate your ability to survive," the Duke of Sussex told crowds in the city's Central Park.
They were followed performances by singer Alessia Cara and pop veteran Cyndi Lauper, who sang a rendition of "Girls Just Want to Have Fun", dedicated to Afghan women.
Click here for more on The Global Citizen festival