A demonstration has been planned against Boris Johnson's purchase of a water cannon.

Metropolitan Police have bought three machines from the German police.

London mayor Boris Johnson ordered the cannons before home secretary Theresa May decides whether to authorise their use.

As a protest, Class War, an activist group, has organised 'Operation Supersoaker' for August 30 in Gravesend.

Spokesman Ian Bone said: "Johnson has bought three second hand water cannons which are shortly to be installed at the Met Police training centre in Gravesend.

"At present it is not even legal to use them on the streets because Theresa May has not approved.

"But why wait – let’s get our retaliation in first.

"There are misgivings in Gravesend about having the water cannon there. Some fear it will attract troublemakers. It has. Us."

The machines have never been used in Britain.

Mr Bone added: "Thousands will flood into Gravesend to take on the water cannon.

"We can finish them off by a daring pre-emptive attack."

The Met has said the cannons will not be used unless authorised by the government.

It is understood the cannons will receive "several weeks" worth of work include fitting CCTV and adaptations to bring them in line with those used in Northern Ireland.