"Had set to with violent thug kicking in bus shelter in Broadway & suffered kick in side for pains," he wrote.
He told the Press Association: "I was walking past and couldn't just ignore it...
"That's when I put my shopping down and went on up and said, 'You can't do that.'"
He added: "I just felt, I live there, it is my city, it is my constituency. When you tolerate that the next thing is that they are smashing up people's faces."
A spokesman for Cambridgeshire Police said Karanovs faced charges of criminal damage, using threatening words and behaviour and common assault and would appear in custody before Peterborough magistrates.