One person died and several were hurt in clashes on Thursday in the flashpoint city of Taez and the capital Sanaa during protests calling for Yemeni President Ali Abdullah Saleh's prosecution, medics and witnesses said.
The bloodletting came as UN envoy to Yemen Jamal Benomar arrived in Sanaa in an attempt to resolve the political crisis that has thrown the country into a spiral of violence in which hundreds have died since January.
"One man was killed and five others were wounded," said a medical official in Taez, Yemen's second city.
Witnesses said Republican Guard troops, commanded by Saleh's son Ahmed, fired artillery rounds into the centre of Taez where tens of thousands of protesters were staging a demonstration to call for Saleh to be brought to international justice.
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Meanwhile, armed civilians opened fire on a similar demonstration in Sanaa, wounding three people, witnesses and medics there said.
"No immunity, no guarantee, Saleh must be tried with his regime," chanted the protesters as they marched on central Sanaa. "World, UN Security Council: Saleh is a war criminal."
The protesters were met with live rounds fired on them from a building as they neared the capital's centre, witnesses told AFP.
Saleh's opponents announced on Friday the creation of a "legal committee" to gather evidence and witness reports from citizens on "the crimes of Saleh and the remnants of his regime" to present to the International Criminal Court.
Peaceful protests have degenerated into battles between rival army troops, security forces and protesters, and between security forces and tribesmen, leaving hundreds of people dead across the impoverished country.
Meanwhile, a defiant Saleh has so far refused to sign a Gulf initiative, supported by the international community, that would see him step down in return for immunity from prosecution for himself and for members of his family.