Dozens had taken to the streets of the Khormaksar district of Aden to protest against the time taken to put on trial prison guards suspected of involvement in the death in custody of Ahmed Darwish, witnesses said.
One protester was killed and another wounded when the army opened fire to disperse the crowd, witnesses said, while a medic confirmed that one person was shot dead.
Darwish was arrested following an attack on intelligence headquarters in Aden in June in which 11 people were killed. Al-Qaeda was suspected of masterminding the attack.
Al-Qaeda is very active in Yemen where its local affiliate has been accused of being behind a spate of deadly attacks on security forces.
Yemen has been rocked by nationwide protests for more than three months demanding the ouster of President Ali Abdullah Saleh, who has been in office since 1978.
More than 145 people have been killed in the unrest.