Eyewitness video posted on social media showed large groups of people gathering and running within the vicinity of a prison in Maputo, the capital of Mozambique.
Shouting was heard and people were seen running in the same direction at the end of the video.
Reuters was able to confirm the location from nearby buildings, utility poles, rooftops and walls that matched satellite imagery. The date could not be verified independently.
A decision on Monday by Mozambique's top court confirming long-ruling party Frelimo's victory in the election has sparked fresh nationwide protests by opposition groups and their supporters who say the vote was rigged.
While police general commander Bernardino Rafael blamed protests outside the prison for encouraging the riot, Justice Minister Helena Kida told local private broadcaster Miramar TV that the unrest was started inside the prison and had nothing to do with protests outside.
About 1,534 people escaped from the prison in the incident but 150 have now been recaptured, Rafael said, adding that there were prison break attempts at two other prisons.
