The starting pistol has been fired for the elite men's race in the Great North Run.
Mo Farah is taking part.
More top news
The problems appear to have been caused by a power supply problem in Queen's Park in north west London, which has affected signalling.
Cloudy conditions continuing overnight, with some patchy outbreaks of rain.
The mayor has revised his proposals for a six-mile East-West cycling superhighway in a bid to cut traffic delays.