Apple's Regent Street store in London has been targeted by a Greenpeace campaign against the company's use of coal power in its data centres.
Activists plastered the windows of the branch with leaflets highlighting the firm's energy consumption, and spent the afternoon speaking to customers and staff about Apple's perceived impact on the environment.
The protest follows reports that Apple allegedly fuels its data centres, including those powering the iCloud service, using 55% coal energy.
Jim Footner, Senior Energy Campaigner at Greenpeace, said: "Apple is a worldwide brand that is renowned for its innovative products that have set the agenda for computers and telecommunications in recent decades, with an immensely loyal customer base. Many of their customers will be surprised and shocked that Apple is using coal to power the iCloud.
"The irony is that Apple has shown that it can be environmentally responsible - their European HQ is exclusively powered by renewable energy. So if they can run their HQ with clean power then they can do it for customers' iClouds."
He added: "If they fail to change course the clean image that Apple has worked so hard to develop over many years will be destroyed."
According to Greenpeace's statistics, if the iCloud service was a country, its energy consumption would rank fifth in the world. The charity predicts that its output will triple by 2020.
Greenpeace praised Google, Yahoo and Facebook for their energy efficiency, and called upon Microsoft, Amazon, Apple and other companies that run cloud services to adopt similar measures.