One of the main Tube lines serving the Olympic Park was suspended in east London on Friday morning as hundreds of thousands of people made their way to events including the first day of athletics competition.
A signal failure in the Bethnal Green area meant the Central line was suspended eastbound between Liverpool Street and Leytonstone, Transport for London (TfL) said.
A TfL spokesman said it was working to fix the problem and had staff at Liverpool Street directing people onto other Tube lines and London Overground services to Stratford.
Events at the park today include Jessica Ennis beginning her campaign for gold in the women's heptathlon.
Despite Friday being the busiest day yet at the Olympics, The Huffington Post UK reporter Luke McGee arrived to see empty seats in the stadium amid the tube chaos.
Empty seats in the stadium after the Central Line suspension
"The Central line is currently suspended in both directions between Leytonstone and Liverpool Street while our engineers fix a signal failure at Bethnal Green, there are severe delays on the rest of the line," a TfL spokesman said.
"We are advising spectators going to Stratford to use the wide range of other lines serving the Olympic Park, including national rail services from Liverpool Street and the District and Hammersmith & City lines, where passengers can use West Ham station with a short walk along the Greenway to the park.
"London Underground tickets are being accepted on local bus services."
The service resumed later, but was still running with severe delays, TfL said.
The breakdown happened hours after the Prime Minister urged people to "come back into the capital" following claims that the 2012 Games had turned London into a ghost town, with commuters and non-Olympic tourists avoiding the city.
There were huge queues for javelin train which goes to the Olympic park from St Pancras in seven minutes
David Cameron said the "threat of meltdown on the traffic system" had been defeated and London was "open for business" during the sporting events.
He told Sky News last night: "People said also that London wouldn't cope, the traffic would grind to a halt, the capital city wouldn't manage. That hasn't been the case either. I think the authorities have done a good job.
"Clearly there is a challenge now, though, to say to Londoners, to the British public who've helped us to, as it were, defeat the threat of meltdown on the traffic system, to say to them now actually there is a case, London's working well, it's open for business, come back into the capital, come and shop, come and eat in London's restaurants and let's make sure that all of London's economy benefits from this."
The stadium eventually filled up despite the travel chaos
Announcements have begun at Tube stations, advising travellers to visit shops, restaurants and other attractions in other parts of London as well as the East End.
"There is a hell of a lot going on and the Tube is there for people to use," he said.
"The Boris announcements (warning of delays) stopped, as they were always planned to do. Now the message is to people to get out there and see what London has to offer."