Relatives of prisoners - convicted or facing charges for their role in Port Said's 2012 football disaster - attacked Sunday the Egyptian Canal city's security directorate after learning the inmates would be transferred to an unknown location.
Rocks and Molotov cocktails were thrown at the building, according to Al-Ahram's Arabic-language news portal.
An interior ministry source told the websit the prisoners had been moved from Port Said prison to provide security and stability to residents living in the area near the facility, which has witnessed fierce clashes of late.
"Thirty-nine prisoners were transported to prisons outside [Port Said] today. The rest of the prisoners will also be transported, as we plan to empty the prison," added the source.
On 26 January, Port Said criminal court sentenced 21 out of 73 defendants to death for their involvement in a football riot that killed more than 70 Ahly fans in February 2012.
The verdict sparked violence that left over 40 dead in Port Said. Ever since, the city has been witnessing mass protests and intermittent clashes.
The court will issue its verdict on the remaining defendants in the case, including nine security officials, on 9 March.