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IMZOUREN, Morocco (Reuters) - Moroccan police firing water cannon clashed with demonstrators in a provincial town on Friday as protests against alleged corruption and abuses of power continued in the most serious unrest since the 2011 Arab Spring upheaval.

A Reuters reporter saw protesters tossing rocks and trash toward riot police in Imzouren, a town 450 km (279 miles) northeast of the capital Rabat. Police responded occasionally with water cannon salvoes.

The clashes came after activist Nasser Zefzafi, the protest leader in the northern town of Al-Hoceima, was arrested at the start of the week and charged with threatening national security, among other offences.

Political protests are rare in the North African kingdom but tensions around Al-Hoceima have been high since October after a fishmonger was crushed inside a garbage truck while trying to salvage his fish confiscated by the police.

Mouhcine Fikri’s death has become a symbol for frustrations over official abuses and revived the spirit of the February 20 movement that spearheaded pro-democracy rallies in 2011 and prompted King Mohammed VI to cede some of his powers.

While some anger in the Al Hoceima protests has been directed at “Makhzen” - the royal governing establishment, the demonstrations in northern Morocco, as in 2011, have not been directed at the king. Morocco has a deeply rooted monarchy, the Muslim world’s longest-serving dynasty.

But the unrest around Hoceima and the Rif region - an area with a long history of dissent against central government - is testing nerves in a kingdom that presents itself as a model for stability and gradual change, as well as a safe haven for foreign investment in a region widely torn by militant violence.