Moroccan police have arrested 40 people in connection with protest movement that has shaken the country's northern Rif region for months.

The prosecutor of the northern coastal city of Al-Hoceima said that those held, including protest leader Nasser Zefzafi, would be investigated for "undermining the security of the state" and other criminal acts.

For its part, the Moroccan Association of Human Rights (AMDH) said on Tuesday it had identified 50 people detained and that "the number of arrests continues to rise" and had passed 70 across the whole province.

The state bears "full responsibility for the consequences of this repressive approach" in the face of peaceful demonstrations in support of "the legitimate demands of the people", AMDH added.

The Rif region has been shaken by social unrest since the death in October of a fishmonger crushed in a rubbish truck as he protested against the seizure of swordfish caught out of season.

Calls for justice for Mouhcine Fikri, 31, evolved into a grassroots movement, based largely in Al-Hoceima, demanding jobs and economic development.

Protestors have come out on the streets every night since Friday in Al-Hoceima, a city of some 56,000 residents.

On Monday night, more than 2,000 demonstrators shouted slogans such as "We are all Zefzafi!" while anti-riot police looked on.

The protest ended around midnight without incident. Demonstrations were also reported in two other northern cities, Nador and Tangiers, as well as in Casablanca, Marrakesh and the capital Rabat, where some 300 people took part.

'Propaganda activities'

Authorities have accused protesters of receiving money and other support from abroad "to carry out propaganda activities".

The mainly ethnically Berber Rif region has long had a tense relationship with Morocco's central authorities, and was at the heart of Arab Spring-inspired protests in 2011.

Interior Minister Abdelouafi Laftit led a large delegation to Al-Hoceima last week, the latest in a series of government trips to the region.

Officials have promised increased support for the local economy, in particular the crucial fishing industry.

Zefzafi, 39, emerged as the leader of the movement by broadcasting passionate speeches online in the local Tarifit dialect from his home or the street, denouncing "corruption" and "dictatorship".