British police are responding to multiple incidents in central London.
Witnesses say a van ploughed into pedestrians on London Bridge on Saturday.
One witness told Reuters she saw what appeared to be three people with knife wounds and possibly their throats cut.
British Transport police say casualties have been reported.
Several witnesses also reported hearing gunshots.
Armed police are also responding to a separate incident at nearby Borough Market.
The incident comes less than two weeks after a suicide bomber killed 22 people at an Ariana Grande pop concert in Manchester.