A suicide bomber has killed at least 45 people when he drove a car laden with explosives into a compound run by local militias in Aden, Medicins Sans Frontieres says.
The official said at least 60 other people were brought into a nearby hospital run by the medical charity in Aden's Mansoura district.
No one claimed responsibility for Monday's attack, but it resembled previous suicide bombings which Islamic State said it carried out in the city.
A security source said the attack targeted a school compound where conscripts of the Popular Committees, forces allied to the internationally recognised President Abd-Rabbu Mansour Hadi, were gathered for breakfast.
The blast rocked the area and sent debris flying, sending residents fleeing, one witness said.
Islamist militants have exploited an 18-month-old civil war between the Houthis and Hadi's supporters and launched a series of attacks targeting senior officials, religious figures, security forces and compounds of the Saudi-led Arab military coalition which supports Hadi.