A passer-by was killed and three others were wounded when police opened fire to disperse protesters in the southern Yemeni city of Aden on Saturday, witnesses and medical sources said. The witnesses said police intervened to clear the roads in the flashpoint Crater district of the port city where dozens of southern activists had set up roadblocks as part of a \"civil disobedience\" campaign. Medical sources confirmed the casualty toll, naming the dead man as Hisham al-Nunu from the city of Taez north of Aden. On Saturday morning, southern activists used rocks and logs to block main roads in several districts, where shops, banks and schools stayed closed, an AFP correspondent said. Normal activity returned to most areas later in the day, apart from Crater where the shootings triggered sporadic clashes between police and protesters, residents said. Pro-autonomy groups have staged street protests every Wednesday and Saturday since February 21 in protest at killings of southern activists in confrontations with security forces. On Friday night, a prison guard was killed and a colleague wounded in a gun attack as a police vehicle from Mansura prison arrived at the local hospital carrying a sick inmate. The assailants fled the scene. On March 18, President Abdrabuh Mansur Hadi warned Yemenis against the use of force to express political views, as he opened a national dialogue to pave the way for the drafting of a new constitution and the staging of elections. The dialogue, scheduled to run six months, brings together 565 representatives of Yemen\'s various political groups -- from secessionists in the south to Zaidi Shiite rebels in the north, in addition to civil society representatives. Most southern factions finally agreed to take part after months of negotiations and under UN pressure. But the movement\'s hardliners led by South Yemen\'s former president Ali Salem al-Baid have dug in their heels, insisting instead on negotiations between two independent states in the north and south. After the former North and South Yemen united in 1990, the south broke away in 1994. The secession triggered a short-lived civil war that ended with the region being overrun by northern troops.