The tear gas was fired as dozens of young people threw stones at the riot police in the alleys of Sidi Abed neighbourhood, where demonstrations have been held for days without any violence.
The northern port city of Al-Hoceima, in the neglected Rif region, has been rocked by social unrest since the death in October of a fishmonger.
Mouhcine Fikri, 31, was crushed in a rubbish truck as he protested against the seizure of swordfish caught out of season and his death has sparked fury and triggered nationwide protests.
In Sidi Abed today, groups of young people gathered again for a surprise demonstration in the alleys of the neighbourhood.
Police responded by forcefully pushing them back to a square where several of them began throwing stones, an AFP correspondent reported.
At least two people were injured, a policeman hit in the jaw by a stone and a demonstrator who had been struck with truncheons.