The Yemeni police forces fired at thousands of protesters marching in the capital to call for outing President Ali Abdullah Saleh, killing at least 12 and injuring more than 50 others, medics said, according to Xinhua.

"More than five bodies of protesters killed by gunshots of government security forces were taken to the opposition-run Technology and Science University Hospital," doctors there told Xinhua.

Earlier, doctors at the field hospital of the Change Square told Xinhua that they received three bodies of protesters, while doctors in the opposition-run Azal hospital said they got four killed by gunshots of government forces.

More than 50 protesters were wounded either by gunshots or smoke of tear gas bombs, according to the doctors.

Witnesses said they heard big explosions in Kintaki intersection, which is located in the southern part of the protesters` epicenter camp.

The attack on the protesters marching through Asar Street, near the headquarters of Saleh`s elite Republican Guards, is still going on as the protesters reached Kintaki area on their way to their epicenter the Change Square.

Meanwhile, sporadic shelling and gunfire took place between the government forces and armed tribesmen loyal to opposition tribal leader Sadiq al-Ahmar in Hassaba district, witnesses and residents said.

No casualties among the rival forces were immediately reported.

However, residents and witnesses told Xinhua that the random shelling between the rival forces injured many civilians, damaged several residential houses and forced a large number of civilians to flee the area.

Blackout hit the capital early morning, according to an official at the Ministry of the Electricity.

Yemen, the impoverished Arab country in the southern tip of the Arabian Peninsula, has faced acute political crisis since the eruption of the protests in late January, demanding to end the 33- year-long rule of Saleh.