Five Yemenis have been killed and at least 22 others wounded in violence in Taez, south of Sanaa, security and tribal sources and witnesses said on Friday.
Security official Colonel Ahmed Rezaz and two companions were killed in a Friday ambush by armed opposition tribesmen, a security source said.
Witnesses said four tribesmen and three civilians were wounded in the ambush in the Sharab al-Rona area of Taez.
Meanwhile, tribal sources and witnesses said two civilians were killed and at least 22 wounded by shelling in northern Taez by Yemen's Republican Guards during clashes with opposition tribesmen.
The fighting broke out on Thursday night between the Guards, loyal to President Ali Abdullah Saleh, and tribesmen, allied with demonstrators who have since January been calling for him to quit, the tribal sources said.
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The shelling was especially focused on Al-Rawda and Osaifra neighbourhoods.
Witnesses said there was widespread panic in the city.
Anti-regime activists had meanwhile appealed for protests under the slogan "Friday of a Civil State," after a call from influential cleric Sheikh Abdel Majid al-Zindani for the establishment of an Islamic state.
Tens of thousands of anti-regime protesters prayed at University Square, the epicentre of protests against Saleh, but did not linger long afterwards, an AFP correspondent reported.
Saleh supporters had called for demonstrations of "Thanks and gratitude to the Custodian of the Two Holy Mosques," in reference to King Abdullah of Saudi Arabia, where Saleh has been hospitalised for more than a month after being wounded in a bomb attack on his palace.
Tens of thousands of regime supporters turned out on Friday, the AFP correspondent said.