Yemeni security forces cordoned off the centre of the southern city of Aden on Friday as they enforced a ban on pro-independence protests that sparked deadly clashes overnight.

Two years after the ouster of veteran strongman Ali Abdullah Saleh, secessionist leaders called for anniversary demonstrations to press their campaign for the restoration of the independence that the south enjoyed until union with the north in 1990.

But a protest march on Aden's central government and diplomatic district late on Thursday prompted police to open fire, killing one demonstrator and wounding 12, medics said.

Video footage posted online showed police firing tear gas to disperse dozens of protesters as they held evening prayers in the middle of a road in Khor Maksar.

Security forces cordoned off the neighbourhood as authorities announced a ban on all gatherings in the area, citing "extraordinary security conditions... and the need to pre-empt terrorist acts."

Aden's top security committee said only peaceful demonstrations with prior authorisation would be tolerated.

But the protest organisers vowed to press ahead with plans for a march on Khor Maksar's Arood Square on Friday in defiance of the ban.

Saleh, who stepped down on February 21, 2012, after 11 months of deadly Arab Spring-inspired protests against his 33-year rule, led the crushing of a southern secession bid in 1994, which resulted in the occupation of the south by northern troops.

Security forces killed eight demonstrators in Aden on the first anniversary of his ouster and Friday's protests were also called to commemorate their deaths.

Saleh's successor, his long-time vice president Abdrabuh Mansur Hadi, had been due to step down on Friday at the end of a two-year transition but his term of office was extended in the face of persistent political unrest.

His government's plans for the unitary state to be replaced by a six-region federation have run into strong opposition both from southern independence activists and from leaders of a decade-old rebellion in the Zaidi Shia northern highlands of the mainly Sunni Muslim country.