The violence erupted late Friday when supporters of the Southern Movement, a separatist group, attacked a march organised by rivals from a year-old protest movement against President Ali Abdullah Saleh, one activist said.
More than 30 demonstrators were injured, some by gunfire, he added.
A medical official confirmed that dozens of people had been hurt, and one who suffered serous head injuries was rushed to hospital in Aden.
A Southern Movement activist blamed the other side for triggering the violence by staging their demonstration in a stronghold of the movement, and said 15 people from his group were injured — nine by bullets.
Nasser Tawil of the Southern Movement said the "tragic and unacceptable" clashes happened because supporters and opponents of Yemen’s presidential election set for February 21 were in the same neighbourhood.
Some factions of the Movement have been campaigning for a boycott of the election, which they say fails to meet their aspirations for autonomy or even southern independence.
Saleh’s deputy, Abdrabuh Mansur Hadi, himself a southerner, is the sole candidate in the election to succeed the veteran strongman who is standing down after more than three decades in power.
Nationwide protests erupted against Saleh’s regime in January last year, triggering months of bloodshed.
Saleh himself arrived in New York on January 28 to receive medical treatment for blast wounds suffered in a June bombing at the presidential palace.
US officials have said he will not return to Yemen until after the election.
Southerners have long complained of discrimination by the authorities in Sanaa, and Tawil accused Saleh supporters of stoking tensions in the south, proposing the election be postponed.
"Why not entrust parliament with guaranteeing Hadi as president to ward off sedition, since Saleh loyalists are likely to repeat the events of Friday night?" he asked.
Ali Salem al-Baid, the exiled main southern leader and former vice president, issued a statement blaming militants loyal to Islamist party Al-Islah for the violence in Aden.
Al-Islah is the most prominent member of the Common Front parliamentary opposition coalition that now heads the country’s national unity government.
Baid accused Al-Islah of getting its northern supporters to stage protests in the south, "pretending in this way that the inhabitants of Aden and the south support the comedy of this election" for president.
