BERLIN, Nov 22 (Reuters) - German police mistakenly beat up their own undercover detectives when a demonstration turned violent in the northern city of Hamburg.

Police were monitoring a rally of some 3,000 people protesting against the demolition of illegal squatters' homes when violence broke out late last Saturday evening.

Several hundred helmet-clad riot police then clashed with the protesters, including plain-clothes officers who had been shoved into the centre of the crowd.

Local political leaders from the Hamburg Social Democrats investigating the matter said the officers tried to identify themselves with a password.

But communication efforts failed and riot police beat their colleagues with batons, leaving two with head injuries.