An audience of 1 million was expected to flood to the Rio de Janeiro Live Earth concert, the only free, open-to-all event of the Live Earth series.
But the Rio state prosecutor's office says there are not enough police to safeguard the climate change awareness concert on Copacabana beach because all security efforts are being concentrated on preparations for the Pan American Games, which start on July 13.
Organisers are now trying to convince authorities to overrule the court injunction suspending the show.
Eight cities are due to hold the concerts on Saturday, aiming to raise awareness about global warming with performances by stars including Madonna, Shakira and the Beastie Boys.
Crowded House, Missy Higgins, Wolfmother and Jack Johnson are among artists set to front Sydney's concert at Aussie Stadium.
Istanbul was last month dropped from the list of cities set to hold the concerts because of a lack of interest and security concerns.
Ticket sales have been slow for most Live Earth shows. Organisers had to shift venues in the South African capital, Johannesburg, in a bid to improve sales.
Officials in Brazil say the injunction on the Rio concert can still be appealed in a higher court.
The municipal RioTur tourism authority, which is involved in the event organisation, says it and show producers have made all necessary arrangements to provide security at the event and are ready to meet any other official demands.
A giant beach stage in front of Rio's plush Copacabana Palace hotel has already been erected.
Organisers say they plan to use dozens of observation towers and platforms, as well as police cameras, in the security scheme for the event.
A cancellation would be embarrassing for city officials, who hope the events will showcase Rio's legendary charms instead of exposing its rampant crime.
Police have stepped up operations against drug gangs in Rio's slums in the run-up to the Pan American Games.
Last week, they raided a slum on the city outskirts, killing about 20 people after a long siege of the area in which 24 people have been killed and more than 60 have been wounded since May.
On Wednesday, police and drug traffickers fought gun battles in two slums, including one by the busy Avenida Brasil thoroughfare. An officer and a bystander were wounded.
Rio has one of the highest murder rates in the world, with a toll comparable to some war zones.
Official figures show at least 1,800 people were killed in the first four months of 2007 in the metropolitan area, official figures show. © 2007 Australian Broadcasting Corporation
