TAIZ/SANAA (Reuters) - Armed men supporting protesters trying to topple Yemen's President Ali Abdullah Saleh killed a security chief and two bodyguards in Taiz Friday, an official said, and four civilians were killed by mortar rounds. Up to 7,000 protesters took to the streets in Yemen's third city that has become the cradle of Yemen's protest movement, some 200 km (120 miles) south of the capital Sanaa, where more anti-Saleh protests took place after Friday prayers. The local government official said the armed men ambushed the car belonging to the area's head of security. Three others in the entourage were critically wounded. Six months of protests against Saleh's three decade-rule have paralysed several cities across Yemen, especially Taiz, where armed members of the opposition have clashed sporadically with government forces as Yemen's political stalemate wears on. While veteran leaders in Tunisia and Egypt bowed to popular demands they quit, Saleh, who is recovering in Riyadh from an assassination attempt, has withstood international pressure on him to step down, leaving impoverished Yemen in political limbo. Local officials said several mortars were fired in neighborhoods of Taiz Friday, where they suspected armed supporters of the opposition to be hiding. But residents and opposition groups said the shelling appeared to be random. They said four civilians were killed and at least seven injured. Further north in Yemen, fighting between Shi'ite rebels known as Houthis and armed members of the country's leading opposition party, the Sunni Islamist Islah raged into their seventh day and have killed some 60 people, the pan Arab daily al-Hayat reported. The clashes bring violence closer to neighboring oil giant Saudi Arabia, which shares a 1,500 km border with Yemen. Riyadh fears unrest could spill over that border. In late 2009, Saudi Arabia launched a military offensive against the Houthis after they briefly seized Saudi territory. Houthi rebels have fought President Ali Abdullah Saleh's government on and off since 2004. Anti-Saleh protests had previously united Houthis and the opposition, but rifts began to appear when armed opposition members took over military and government sites.