Yemeni police said they shot dead three people who were heading to a rally for southern independence on Thursday, as the deeply divided country marked a year since the ouster of strongman Ali Abdullah Saleh.
They said a passer-by was also killed when, according to Southern Movement member Fathi Ben Lazraq, "they fired on activists trying to reach the place where the rally was being held" in Aden's Parade Square.
Reports said police were trying to prevent clashes between them and the Islamist Al-Islah (Reform) party, which held a demonstration in the same square in support of national unity and of interim President Abdrabuh Mansur Hadi.
A Southern Movement leader, Naser al-Khabji, said that "clashes could break out" if pro-Hadi supporters demonstrated.
Security sources said police also wounded 34 southerners in clashes around Aden, with the Southern Movement saying they were trying to prevent protesters from entering the southern port city from neighbouring provinces.
And two policemen were hurt by sniper fire from the rooftops of buildings surrounding the square, security officials said.
Aden was paralysed as security forces deployed heavily around the city.
Thousands of Al-Islah supporters waved Yemeni flags and held portraits of Hadi as well as banners reading "unity is our strength," chanting "for dialogue, we will pursue our march".
That was an allusion to a delayed national dialogue aimed at drafting a new constitution in readiness for presidential and parliamentary elections in February 2014 that would end a two-year transition period.
Southern Movement leaders have said they are ready to join the dialogue, but a hardline separatist faction led by exiled Ali Salem al-Baid has refused to take part.
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Abdullah al-Alimi, organiser of the Al-Islah rally, said the "cause of the southerners is just, but it should be resolved through dialogue".
For their part, southern activists carried flags of the former South Yemen, which was a separate state before unification with the north in 1990.
They also displayed pictures of Baid, who served as the last president of the region before union, and leads a hardline, pro-independence faction of the Southern Movement.
"Revolution in the south; occupiers go out," they chanted.
South Yemen broke away in 1994, sparking a civil war, before it was overrun by northern troops.
Saleh was formally replaced by Hadi on February 21, 2012 after a year-long deadly uprising to oust him, under a UN-backed power transition agreement brokered by the Gulf Cooperation Council that called for a two-year transition.
Some factions of the Southern Movement want autonomy for the area, but more hardline members are pressing for a return to complete independence for the south where residents complain of discrimination by the Sanaa government.
During a speech in Aden to mark one year in power, Hadi promised that elections under a new constitution would be held next February to usher in a "new era of stability".
He also denounced "calls for armed conflict, which will lead to the loss of the southern cause," in an apparent reference to Baid's faction.
The increasingly restive south has been hit not only by the Southern Movement's campaign for self-rule but also by deadly clashes between the army and militants loyal to Al-Qaeda.
In his attempt to unite the country, Hadi also faces interference from partisans of Saleh, who ruled for 33 years.