The shelling hit a small fishing harbour and a barge carrying civilians, said the official who gave the toll, blaming Huthi rebels battling to take control of Aden's Tawahi neighbourhood and clashing with forces loyal to exiled President Abedrabbo Mansour Hadi.
"Dozens of civilians were killed in this massacre committed by the Huthi rebels and fighters loyal to ousted president Ali Abdullah Saleh," said Ali al-Ahmadi, a spokesman for the southern pro-Hadi Popular Resistance Committees.
Witnesses have said the rebels have intensified attacks over past days on Tawahi, which houses the headquarters of pro-Hadi Aden TV.
The commander of troops loyal to Hadi was shot dead during the battles today.
General Ali Nasser Hadi, who is not related to the exiled president, was appointed in March to lead loyalist forces in a southern area including the provinces of Aden, Abyan and Lahj.
Pro-Hadi forces, including military units and militia fighters, have been battling the Iran-backed rebels for weeks in Aden, the president's former stronghold before he fled to Riyadh.