The protesters died after police fired on demonstrators who had wanted to hold a public funeral procession for a southerner whose death in police custody last month stoked tensions in the port city. Police said some of the crowd were armed.
"The police tried to prevent them from organizing an unlicensed march, but they attacked the police. The police responded by using gas bombs," a police official said.
"But some of the elements were armed, so clashes happened and one of the protesters was killed," he said. Three other protesters were wounded.
The Arab state has seen increasing tit-for-tat violence in its south in recent months as a small group of armed separatists mounts what analysts describe as a budding insurgency, two decades after the unification of North and South Yemen.
Tension has been high in Aden since authorities launched door-to-door raids on June 25 in search of Al-Qaeda militants wanted in connection with an attack on the city's intelligence headquarters that killed 11 people.
One person detained in the manhunt died. Security officials said he suffered an asthma attack, while opposition website Sahwa Net said he died from wounds after being tortured.
The death caused neighborhood protests and renewed a cycle of violence that often draws in separatists, who say that the government discriminates against southerners and responds to shows of dissent with disproportionate force.
The protesters who clashed with police on Wednesday had tried to retrieve the body of the detainee from an Aden hospital when police barred them from proceeding.
"The police blocked the path of the mourners and used live ammunition immediately," a separatist official said, adding that the violence then prompted stone-throwing protests.
The attempt to hold the funeral procession on Wednesday coincided with a separatist "Day of Rage" marking the anniversary of a 1994 separatist rebellion that was quelled by the government.
