Yemeni riot police shot dead a protester and injured five others when they opened fire on a march of thousands of demonstrators in the capital Sanaa today.

Protesters in Sanaa marched early in the morning from the University of Sanaa to the Ministry of Justice while chanting, "the people want the fall of the regime".

Riot police backed by plain-clothes officers confronted the rioters, opening fire and throwing stones.

A medical official said one man was shot in the neck and killed.

Meanwhile residents of Yemen's port city Aden said security forces have disappeared from the streets, threatening to send the city into chaos.

Residents say groups of men are looting and burning government buildings and there is no sign of police or armed forces.

Over the past nine days of protests, seven people have been killed and more than 150 wounded as police fire tear gas and gunshots.

Demonstrators are calling for the removal of President Ali Abdullah Saleh - a key US ally in fighting al-Qaida - who has ruled the country for 32 years.

He is already facing a restless population, with threats from al-Qaida militants who want to oust him, a southern secessionist movement and a sporadic armed rebellion in the north.

To try to quell the new outbursts of dissent, Saleh pledged to meet some of the protesters' demands and has reached out to tribal chiefs, who are a major base of support for him.

But a key chief from Saleh's own tribe was critical of his policies and threatened to join the protesters - an apparent attempt to pressure the embattled leader of the world's poorest Arab country.

Most of the protesters appear to be students, educated professionals and activists who used social media sites Facebook and Twitter in summoning people to the streets.