ANTANANARIVO (AFP) - Two Malagasy men were on Thursday charged over last week's lynching of two Europeans accused of sexually abusing, killing and mutilating a young boy on a tourist island.

Eleven others are being charged for rioting and attacking police barracks the night before the pair - a Frenchman and a Franco-Italian - were mobbed by locals and burned on a beach ringed by bars and hotels on the island of Nosy Be.

"Of the 13 people that appeared before the judge, two are involved in the murder of two foreigners and the other 11 are involved in the attack on the barracks of the gendarmerie," police deputy commander Guy Bobin Randriamaro told a news conference.

A local man was also killed by a rampaging mob hours after the foreigners were slain but it is unclear whether his suspected murderers have been arrested.

In all 37 people have been detained in connection with the October 3 violence but 24 have already been freed.

Nosy Be has been plagued by sex tourism but a neighbourhood leader said the eight-year-old local boy's body was wet when it was found, suggesting that he may have drowned.

Mob justice is common on the vast island nation off the southeastern African coast, which authorities struggle to police effectively. French police said it was launching its own probe.