First reports suggested a power surge on the London Underground stopped trains and possibly caused a collision. Since then I've heard:
whole Underground has been closed
About 30 deaths according to eyewitness who had spoken to police
An eyewitness said a litterbin had exploded
Recently, there have been reports of a bus exploding.
Watching it on the bbc website, listening on 5 live
Too early to say what is really going on.
UPDATE: National Gird have confirmed there was no power surge. Areas have been cordoned off, fatalities have been reported.