Rachid Benali told AFP the verdict was issued on Tuesday by a court in Al-Hoceima, a port city in the neglected northern Rif region which was rocked by protests for social justice.
The court sentenced two members of Hirak al-Shaabi, or “Popular Movement”, to 20 years in jail and four others to prison terms ranging from one to five years, Benali said.
They were accused of having torched a building that housed policemen in March, in the city of Imzouren near Al-Hoceima.
No one was injured in the fire which damaged the building and forced the evacuation of dozens of policemen from the roof.
In August another activist was also sentenced to 20 years in jail in connection with the arson attack.
Northern Morocco was rocked by months of protests after the death in October last year of a fisherman who was crushed in a rubbish truck as he tried to retrieve swordfish confiscated by authorities because they were caught out of season.
The protests demanding justice for 31-year-old Mouhcine Fikri quickly snowballed into a wider social movement led by Al-Hirak, demanding jobs, development and an end to graft.
In response, security forces launched a crackdown, putting the alleged leaders of the mainly young protesters in jail in May and June.
Hirak leader Nasser Zefzafi was among those arrested.
His trial, alongside 53 co-defendants, opened at the end of October.