Mouhcine Fikri, 31, was accidentally killed last Friday in the northern city of Al-Hoceima as he tried to protest against the seizure and destruction of swordfish, which are not allowed to be caught at this time of year.
His death in the Rif -- an ethnically Berber region long neglected and at the heart of a 2011 protest movement for reform -- also triggered anger in other cities including the capital Rabat.
"Mouhcine we won't abandon you!" protesters cried in Al-Hoceima, according to footage streamed live on social media, holding up photos of Fikri and waving Berber flags.
Fikri's case "concerns all Moroccans suffering from oppression", one woman cried out to fellow protesters by loudspeaker.
"We have come to protest against unfairness, against a corrupt system," she said.
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Another protest was held in the Rif town of Nador, according to social media.
King Mohammed VI was quick to order a "thorough and exhaustive investigation" into Fikri's death and sent the interior minister to offer condolences to his family.
Authorities earlier this week arrested 11 people suspected of involuntary manslaughter over Fikri's death.
Of those, two interior ministry employees, two fisheries officials, the head of the local veterinary services and three rubbish collection workers were remanded in custody on Tuesday.
It remains unclear who activated the truck's crushing mechanism that killed Fikri.
Thousands attended Fikri's funeral in Al-Hoceima on Sunday after an image of his body -- head and arm sticking out from under the lorry's crusher -- went viral on social media.
The self-immolation of a street vendor in late 2010 in protest at police harassment sparked Tunisia's revolution and the Arab Spring uprisings across the region the following year.