Kraftwerk fans have brought the Tate Modern's website to a standstill after a rush to buy tickets for a retrospective of their albums in February.
The German electro pioneers are due to play eight shows in the Turbine Hall, playing an album in full each night.
But Tate's website crashed under the demand when booking began at 07:30 GMT.
Lines were then jammed after the gallery advised fans to try booking via its telephone ticketing line.
By 08:50 GMT, the ticket page on Tate's website said: "Kraftwerk at Tate Modern has just gone on sale and we are experiencing a phenomenal demand for tickets which is affecting our web server.
It added: "We have extra staff on hand today but demand is extremely high."
Despite fans complaining about a five-hour struggle to purchase tickets, Tate director Nicholas Serota defended the sales process.
"This isn't the first website to crash through over-demand," he told the BBC. "The Olympic site did something similar."
He added: "There are a limited number of tickets, there is a huge demand and I hope we can satisfy a good many of them."
The gallery also posted messages apologising for the problem on its Twitter page. But furious fans responded their own messages.
Pete Gleadall tweeted: "No it's not working neither is the website. If you can't cope with demand get some professional ticket sellers who can."
While Steve Philips wrote: "'Please try to buy tickets online again later' - when? What's stopping you selling them online now?"
Lynne Dellar posted on Tate's Facebook page: "I've been on the phone for 2 hours now. The redial button is almost worn out. It would be nice to know if I'm wasting my time so I can go and get some breakfast,"
Some fans have also said they had gone to Tate Modern in person, to try and buy tickets.
A spokeswoman for Tate said: "Tate's IT and ticketing services teams worked closely together and had procedures in place to ensure that its IT systems and website had the capacity to manage a high volume of demand.
"Unfortunately we are experiencing problems with our server which is why we are directing people to the phone lines."
She added: "Tate sells millions of tickets a year through its web systems and for this event there are a minimum of 900 tickets on sale for each night."
Kraftwerk were at the forefront of synthesiser pop and had a huge influence on hip-hop and dance music in the late 1970s and early 1980s.
Kraftwerk: The Catalogue is described as a "chronological exploration of the group's sonic and visual experiments" and promises "spectacular 3D effects".
It was first performed at New York's Museum of Modern Art earlier this year.
Beginning with their fourth album, 1974's Autobahn, the quartet will also play Radio-Activity (1975), Trans Europe Express (1977), The Man-Machine (1978), Computer World (1981), Techno Pop (1986), The Mix (1991) and Tour de France (2003), along with additional compositions from their back catalogue.
The shows will be the group's first in London since 2004.