Detectives suspect a serial attacker could be behind a string of vicious assaults on older men in two parks.
Three men, aged between 63 and 84, were injured in the attacks, all in Muswell Hill, north London.
Two of the victims remain in a critical condition in hospital after apparently motiveless assaults in Alexandra Park and Queens Wood.
Investigators from Scotland Yard's Homicide and Serious Crime Command have been brought in to catch the man responsible.
A Metropolitan Police spokesman said a man has been arrested in connection with the attacks and was in custody last night.
Police are struggling to explain the motive behind the attacks. Two of the men were walking their dogs and one man was carrying 700 but was not robbed.
Detective Chief Inspector Tim Duffield, who is leading the inquiry, said all three attacks had similar features.