Mate of mine drove past the incident just after it happened and posted on Facebook that pedestrians are down on *both* sides of the bridge so he said a car attack looks unlikely.
According to a BBC reporter on the bridge at the time, and still reporting from there on television news, it was a white van with just the driver inside. From what I can understand, he got away. All a bit sketchy at the moment.
Meglet, the BBC reporter said a white van ploughed into people. She said several people are injured and police boats are on the river presumably searching for victims who are thought to have been knocked into the water.