Tube trains to the Olympic Park have been suspended.
Spectators travelling to Stratford on the Central line at around 7am were told all services were terminating at Liverpool Street due to an incident at Leyton.
A defective train westbound was awaiting the arrival of the London Fire Brigade, a train driver said.
Station announcers at Bank said there were reports of a fire on board the train.
The fire alert meant there were no Central line services between Liverpool Street and Woodford/Newbury Park and there were severe delays on the rest of the line.
Early-morning travellers on the Tube were being advised to take the District or Jubilee lines to Stratford instead.
To add to the Olympic travel problems, rail services in and out of Liverpool Street main line station were disrupted due to an overhead wire problem between Elsenham and Stansted Mountfitchet in Essex.
This was affecting travellers on services run by the Greater Anglia and CrossCountry train companies.
The problems around Liverpool Street come on another busy day for public transport as the first weekday evening football match takes place at Wembley Stadium. This is only one of a number of Olympic events taking place in the capital, with Transport for London (TfL) advising that London is expected to be exceptionally busy throughout the day.
TfL said people were heeding advice to avoid London if possible. It added that while Tube journeys were up 4% on Monday, road journeys in London were down by 20%.