A German man who was blinded by a water cannon in Germany has come to the UK to warn against their possible use in London.
Dietrich Wagner - a 69-year-old retired engineer - was hit in the face at a protest in Stuttgart four years ago. His eyelids were torn and some of the bones around his eyes fractured, causing his eyeballs to fall out of their sockets.
Mr Wagner's injuries are very rare but he has travelled to the capital to lobby the mayor, Boris Johnson, who is asking Londoners whether the Metropolitan Police should have water cannon available in future.
Mr Wagner was attending a demonstration over the redevelopment of Stuttgart's main railway station on 30 September 2010. He remembers the jets on the water cannon suddenly becoming more intense: "I felt something like a punch in my face and then I fell backwards," he recalls.
'A tool of violence'
He lost consciousness for a while and later woke up as two fellow protestors were carrying him: "I cried out several times that I could not see and I could only see black. I was not aware that my eyes were hanging out [of their sockets]."
There are graphic photographs of what happened to Dietrich Wagner on the internet. He needed six operations and a metal plate was inserted in his head as part of the reconstructive surgery. In one of his eyes, he only has 8% sight.
His injuries were extreme, but his case is being used by those who want to prevent the introduction of water cannon to the capital.
Dietrich Wagner says he hopes his appearance in London this week, including a visit to City Hall, will help sway the Met against deciding to buy any water cannon. "It's not a democratic tool," he insists, "It's a tool of violence."
A spokesman at Scotland Yard responded to Mr Wagner's visit by saying: "No tactics that we use can ever be entirely risk-free. The options available to us at the moment actually involve a higher degree of force - such as baton rounds, horses, dogs or vehicle tactics."
The Metropolitan Police became interested in the idea of purchasing three second-hand water cannon from Germany after the English riots of August 2011.
The Home Office was reluctant to provide the necessary funding but London's mayor has been more sympathetic to the idea. Mr Johnson is carrying out a public consultation which ends in two weeks. Any final decision will still lie with Home Secretary Theresa May.
Last week peers in the House of Lords held a lively debate about whether water cannon should be authorised outside Northern Ireland.
Speaking about their use there, the Ulster Unionist Lord Empey argued they could be useful when separating rival groups of people.
He warned critics that they shouldn't be superior about the UK model of policing. "Public disorder will change in character over the years," he said. "We are not in the era of Dixon of Dock Green any more."
There's also a big debate within policing. The former Met Police Commissioner, Lord Blair, has said he believes a good case has not yet been made for the use of water cannon outside Northern Ireland.
A briefing paper for the Association for Chief Police Officers said water cannon could be "capable of causing serious injury or even death" but it went on to point out that they had been used in Northern Ireland for the last decade without any recorded injuries.