At least 21 people have been killed in unrest after Mozambique's top court on Monday confirmed long-ruling party Frelimo's victory in the election, the country's interior minister said late on Tuesday.

The decision by Mozambique's Constitutional Council sparked fresh nationwide protests by opposition groups and their supporters who say the vote was rigged.

Seventy-eight people have been arrested so far and security measures have been tightened across the country, Interior Minister Pascoal Ronda told public broadcaster TVM.

"The armed and defence force will increase its presence in critical and key points," he said.

Frelimo has been repeatedly accused by opponents and election observers of rigging votes. It has denied those accusations.

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