SANAA — Troops on Sunday killed at least one protester demanding the trial of Yemeni President Ali Abdullah Saleh for crimes committed during his decades-long rule, hours before he was expected to make a key speech.

The protester was shot in the head as he shouted into a megaphone while perched on top of a minivan that was leading the march of tens of thousands of people in the capital, an AFP correspondent reported.

A medical official at a makeshift field hospital in the city said 17 other people were wounded in the attack.

The protesters, who chanted "Freedom! Freedom! The people want the butcher tried!" retreated to their base in Change Square, epicentre of anti-regime protests in the capital, after the shooting.

Sunday's demonstrations came after the bloodiest week Sanaa has seen since mass anti-government protests calling for Saleh's resignation erupted in January.

In all, more than 170 people, mostly unarmed protesters, have been killed in the capital over the past week.

Most of the casualties were anti-government protesters killed when security forces used artillery and gunfire to disperse demonstrating crowds.

In Yemen's second largest city of Taez, meanwhile, three people were killed and at least seven others were wounded in clashes on Sunday.

Two tribesmen were killed and three others were wounded overnight when fighting erupted between tribesmen who have thrown their support behind anti-government protesters and security forces loyal to Saleh, a tribal source told AFP, requesting anonymity.

A third man was shot dead by government troops, a medical official said.

In a separate incident on Sunday, witnesses said that the army bombarded central Taez, wounding four people when their car was hit in the shelling.

The clashes came a day after security forces in Taez, including the elite Republican Guard troops commanded by Saleh's son Ahmed, bolstered their deployment throughout the city and on its outskirts.

The beefed up military deployment came after Saleh's return to Yemen on Friday after a more than three-month absence in Saudi Arabia for treatment for wounds sustained in a June explosion at his presidential compound.

Taez has been the scene of intense clashes between government troops and anti-government protesters since calls for Saleh's resignation first began.

At least eight people have been killed there in the past week, six of them civilians, according to medical officials.

The latest protests in the capital Sanaa came amid renewed calls by the United Nations, United States and Gulf leaders for Saleh to step down and transfer power to Vice President Abdrabuh Mansur Hadi.

The Gulf Cooperation Council nations, sponsors of the Gulf initiative that provides a road map for a peaceful transition of power in Yemen, have been rebuffed by Saleh, who has repeatedly delayed signing the initiative.

On Saturday, GCC ministers condemned the violence in Yemen and echoed US and UN calls urging Saleh to "immediately" sign the initiative.

They also called for "self-restraint, a complete and immediate ceasefire, and for forming a commission of inquiry in the latest events that have cost the lives of innocent Yemenis."

A UN Security Council statement on Saturday called on all sides to "reject violence, including against peaceful and unarmed civilians, and show maximum restraint."

"They called on all parties to move forward urgently in an inclusive, orderly and Yemeni-led process of political transition," it said.

Meanwhile, government forces, dissident troops and armed tribesmen, both pro- and anti-Saleh, remained heavily deployed throughout Sanaa, an AFP correspondent said, raising fears of renewed clashes ahead of Saleh's expected speech.

The official Yemeni news agency said he would make "an important speech to mark the 49th anniversary" of the September 26, 1962 revolution that saw Yemen proclaimed a republic, although no appearance has been officially announced.

Yemenis are sceptical that a Saleh speech will end the violence.

"We are living in a state of fear," one Sanaa resident who identified himself only as Fuad told AFP. "I don't think there's anything Saleh can say that will change that."