Yemeni troops opened fire Thursday on thousands of people demonstrating in Taez for President Ali Abdullah Saleh to step down, wounding at least 10 people, the protesters said.

Live bullets were fired into the crowd by members of the elite Republican Guard, which is headed by Saleh's son Ahmad, and local security forces in Taez, the country's second city, they said.

Medical officials who spoke to AFP on condition of anonymity confirmed the casualties, adding three of the wounded were in "critical condition".

Taez has been the scene of sporadic clashes between government troops and forces deployed since June by influential tribal leaders to protect pro-democracy protesters from attacks by Saleh loyalists.

The renewed violence comes just one week after President Ali Abdullah Saleh authorized his deputy to negotiate a power of transfer agreement with opposition leaders.

A truce was reached last month after a previous ceasefire agreed upon in June collapsed.

In Sana’a, a source in the office of leading tribal chief Shaikh Sadok Al Ahmar said three people were killed when a tank shell struck a house in the capital's Al Hassaba district earlier on Thursday.

Two civilians were killed immediately by the blast while the third victim died later of injuries sustained in the explosion.

Following the strike, violent clashes broke out between Saleh's forces and dissidents loyal to Ahmar, who supports protester calls for Saleh to step down from power.

Two of his fighters were also wounded in the overnight violence.

The head of the UN human rights office, Hanny Megally, warned on Wednesday that Yemen is heading towards civil war.

Saleh, who has ruled Yemen for 33 years, has lost his grip on power since nationwide protests erupted calling for his resignation in January.

He has been in Saudi Arabia since June, recovering from a bomb attack targeting him in his presidential compound in Sana’a.