SANA'A: Yemeni security forces opened fire on a massive anti-government demonstration in the capital Sana'a Wednesday, killing 12 protesters and wounding some 190, a doctor at the scene said. The violence broke out as about 100,000 government opponents filled a landmark square at the epicenter of the uprising, spilling into the streets around the state TV building. Witnesses said security forces, including members of the elite Republican Guard, fired live ammunition and tear gas into the crowd to break it up. Snipers were seen on nearby rooftops aiming at the crowd. “Many of the dead and wounded were shot in the head and torso,” doctor Mohammed Al-Ibahi said. Yemenis in at least 18 cities and towns launched a civil disobedience campaign Wednesday in an escalation of their more than two-month-old uprising to bring down long-serving President Ali Abdullah Saleh. Shops, schools and government offices were shuttered. The closures are planned twice weekly until Saleh steps down, activists said. Inspired by revolts across the Arab world, Yemenis have staged near-daily protests calling for the ouster of Saleh, the country's ruler of 32 years. At times, millions have flooded the streets of the capital and other cities and towns. The president has clung to power despite the street protests and defections by many loyalists, including his tribesmen, military officers and senior officials. Security forces and Saleh supporters have killed more than 130 people since the unrest erupted in early February. Chaos ensued after government forces opened fire in Sana'a Wednesday, sending protesters scattering in all directions to escape sniper fire and security agents hurling rocks.