LONDON, England (AP) — Severe delays were reported this morning on a busy subway line serving Olympic venues, on a day that heavy crowds are expected as the track and field events get under way.

Transport for London said service on the Central Line, which links the centre of the city with the Olympic complex, had been briefly suspended because of a signal failure. Officials said full service was resumed by 9:00 am after a partial suspension that lasted more than an hour, but there were still delays in the line mid-morning.

The delays have a knock-on effect throughout the system as commuters and Olympics fans switch to other lines.

The problems developed on what officials expect to be one of the busiest days at the Olympics Park in east London, with some 200,000 people anticipated for a variety of events. The heavy visitor numbers are expected to strain the transport network, which depends on a variety of train lines being able to reach the park.

Today marks the first day of events inside the 80,000-seat Olympic Stadium, which had so far only been used for the gala opening ceremony.

Network Rail officials said the Javelin train linking central London to the Olympic Park was working properly Friday morning.

Officials are also restricting access to an oversize shopping mall next to the Olympic Park today and Saturday because of the large crowds expected. Only ticket holders or accredited workers will be allowed into the mall during the day.