Defence lawyers for the activists from the Initiative for the Resurgence of the Abolitionist Movement (IRA) had called this week for the case to be thrown out saying their clients were tortured in police custody.
The activists were intially sentenced to between three and 15 years at their trial in the capital in August, on charges of using violence, attacking security forces, gathering while armed and membership of an unrecognised organisation.
"The prosecutor demanded 20 years of imprisonment against the whole group," a judicial source told AFP on Wednesday.
The prosecutor argued that the activists had "directly taken part in the events", and accused them of "rebellion against the police and vandalism", the source added.
Local officials allege about 10 police officers were injured during the demonstration by a Nouakchott slum community that was being forcibly relocated as the city prepared for an Arab League summit on July 25.
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The slum was home to many so-called Haratin -- a "slave caste" under a hereditary system of servitude whose members are forced to work without pay as cattle herders and domestic servants, despite an official ban.
Seven of the slum's residents are also appealing against their sentences.
One of the accused Balla Toure, vice-president of the IRA, denied having used violence.
"I'm a development worker and not a destructive person," Toure told the court, according to local media.
The appeal hearing in Zouerate in the north of the country should be completed by next week, according to a legal source.
Rights group Amnesty International have called the trial a "farce" and French lawyers for the accused have filed a criminal complaint in Paris for "torture and cruel treatment".
