Aden: Yemeni security forces fired live ammunition and tear gas at separatist demonstrators in the southern port city of Aden on Monday, leaving some of them injured, witnesses said.
Supporters of the Southern Movement, which wants independence from Sanaa, had called for a "Day of Rage" and blocked roads across Aden.
South Yemen was independent between the end of British colonial rule in 1967 and 1990, and a succession attempt four years later sparked a civil war, before it was overrun by northern troops.
Southerners still complain of discrimination by the Sanaa government.
On Monday, the demonstrators in Aden clashed with police, and some of them suffered injuries, according to witnesses, who were unable to put a figure on the casualties.
Armed supporters of the Southern Movement were meanwhile suspected of being behind the kidnapping of five Yemeni soldiers in the neighbouring province of Dhaleh, a government official said.
The motive of the kidnappers was to pressure the government into releasing a Southern Movement member who had been arrested in Aden last month, the official told AFP.
Yemen has been rocked by growing instability since a year of bloody anti-regime protests forced long-time president Ali Abdullah Saleh to leave office in early 2012.
Shiite rebels from the north overran Sanaa in September and have expanded their presence throughout Yemen in the face of fierce resistance from Sunni tribes backed by Al-Qaeda.
A coalition of two separatist groups has urged southerners working for the government, especially those in the armed forces and police, to abandon their jobs and join the protests.