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 on Friday include Jessica Ennis beginning her campaign for gold in the women's heptathlon.
"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," the 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."