There were clashes in Yemen on Thursday over the country's 1990 unification - part of ongoing protests that have threatened to reopen the country's old north-south divide.
At least four people died in the violence, which broke out in the capital Aden a day before the anniversary of the union - one marked by unity celebrations, political speeches and military parades.
Dozens more people were wounded when police opened fire on the thousands of anti-government protesters.
A security official in Aden said police shot into the air to disperse the crowd, but did not confirm or deny any deaths or injuries.
He emphasised that the protest had not been officially approved.
The protests are reportedly being organised by the Southern Movement, a loose coalition of groups opposed to the government in Sanaa, the capital.
Protesters sang independence songs, highlighting the depth of separatist sentiment in the south where living conditions are worse than in the rest of the country.
But in his anniversary-day address, Ali Abdullah Saleh, the Yemeni president, gave warning that those who want to secede would fail as they did in 1994.
He said the country's unity was "solid and blamed "outlaws" for the violence, which he said aimed "to hit at the nation and its safety and to stir unrest".
Saleh has cautioned in the past against the risk of Yemen splitting into "several entities".
In addition to the southern unrest, Yemen's government also faces the risk of a potential rebellion by Shia Zaidi Muslims living in the north.
Saleh announced during his speech that his government had plans to change the constitution and develop the political and electoral system so as to grant more power to provincial rule in an effort to attract investment.
Southern Yemen is home to four million people, whereas the north's population is more than 20 million.
For decades southerners have complained of jobs and land being reserved for the northerners.