A suicide bomber gravely wounded three members of the separatist Southern Movement in an attack in the southern Yemeni province of Lahij on Friday, the group and a local official said.
The bomber detonated an explosives belt as an activists' cars headed for the port of Aden for a demonstration on Saturday marking the independence of the former South Yemen.
The bomber was killed "and three members of the Southern Movement were gravely wounded and evacuated towards Aden to be hospitalised", a Southern Movement source said.
A local official confirmed to AFP that three people had been seriously wounded in the attack, for which there was no immediate claim of responsibility.
The southern question has been a major stumbling-block in a Yemeni national dialogue launched in March, with hardline factions of the Southern Movement demanding secession and boycotting the talks.
Secessionists have called for a major rally to be held in Aden on Saturday in support of their demands.
Elsewhere in Yemen, an army colonel was shot dead by two gunmen riding a motorcycle in the eastern Hadramout province, where Al-Qaeda in the Arabian Peninsula is active, a security source told AFP.
Yemeni security forces have faced increasing attacks in recent months that the government blames on the local Al-Qaeda franchise, widely seen as one of the most sophisticated offshoots of the global terror network founded by Osama bin Laden.
AQAP has taken advantage of the weakening of the central government since a popular uprising that toppled president Ali Abdullah Saleh in 2011.
Washington regards AQAP as the global jihadist network's most dangerous affiliate and has stepped up drone strikes targeting the militants.