The authorities say Ouarzazate prison in central Morocco recorded 303 cases, while 10 other cases were in Oudaya prison in Marrakesh and Ksar Kebir prison in the north-west.

Most of the cases involved prisoners, but a small number of prison warders were also diagnosed.

The mass testing started after one person in Ouarzazate prison tested positive last week.

The authorities say they have isolated all positive cases and all warders have been issued with protective gear.

There are nearly 80,000 inmates in Moroccan prisons. In early April, more than 5,654 inmates were pardoned by the king to reduce the risk of spreading the virus in notoriously overcrowded prisons.

Morocco has 4,252 confirmed cases of coronavirus, including 165 deaths.