Shots were reported to have been fired close to the Houses of Parliament, the BBC reported.
Politicians and journalists tweeted about hearing loud crashes outside the buildings.
Witnesses said they saw people being treated for injuries and reported seeing a man with a knife in the palace grounds. Staff inside Parliament were told to stay inside their offices, the report said, adding someone had been shot.
Scotland Yard said it was called to a firearms incident on Westminster Bridge amid reports of several people injured.