At least a dozen people have been injured on Westminster Bridge after shots were heard outside the British parliament, a Reuters photographer said on Wednesday.

There were reports of politicians and journalists tweeting 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.

More details are awaited.