One of the main Tube lines serving the Olympic Park was suspended in east London this 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.
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.