At least four protesters were shot dead Tuesday in clashes with Yemeni security forces who opened fire to intercept a demonstration in the capital Sanaa, demanding the ouster of President Ali Abdullah Saleh, medics and witnesses said.

"Four protesters have so far confirmed dead and up to 40 others were wounded, including a woman whose condition is critical," a medic at a field hospital at the protesters' Change Square in Sanaa told Xinhua.

The government forces also used tear gas, water cannon and hired "thugs" wielding heavy weapons to disperse the protesters who threw stones at the security forces, according to the witnesses.

"One of the dead protesters was hit by a rocket-propelled grenade," the medic added.

About one hour after the clashes, the protesters went back to their base camp of "Change Square" outside Sanaa University.

Saleh who has been in power since 1978 refused to step down despite pressure from protesters and several foreign governments.

Local observers warned that the impoverished Arab country recently appeared to enter a new phase of dangerous violence.

The warning came days ahead of an expected United Nations Security Council resolution on the months-long political crisis in Yemen.