All people in France who leave home for any reason during the current lockdown to combat the coronavirus pandemic must fill in an official form, declaring their reason for appearing in public.
Police in the western town of Lannion said that in the early hours of Saturday the man, 39, was found lurking behind a car by a police patrol and asked what he was doing.
"He explained that he was waiting for someone to 'smash their face in' and was carrying a flick knife, local police commander Daniel Kerdraon told the AFP press agency.
Sticking to the letter of the law
"He had the written form with his real name and the time he had left home -- 10:15 pm. But we told him that the reason for going out was not valid.
"In his own way, he was trying to fulfil the letter of the law," the police officer added.
The man, who had been drinking, was detained overnight and released Saturday morning with a mandatory 135 euro fine for breaking the lockdown rules, and another 150 euro fine for drunken behaviour.
"We showed him how to fill the form properly and made clear that going out to 'beat someone up' is not a valid reason," Kerdraon said.
The man will be questioned again on Monday for carrying a weapon.
