Witnesses say Yemeni security forces and snipers have opened fire on thousands of anti-government protesters marching to the Cabinet building, killing at least six and injuring at least 40, medical officials and protesters said.

The protesters calling for the ouster of longtime President Ali Abdullah Saleh were marching from a main square toward the Cabinet when they came under fire from snipers on rooftops, plainclothes security forces, and soldiers with anti-aircraft guns mounted on pickup trucks.

"The snipers were shooting at the people," Talal al-Hamadi, a protester, said. "People rushed and some fell over each other. There was a stampede."

Earlier Wednesday, security forces fired on anti-government protesters in southern Yemen, killing three people, as demonstrators took over an Oil Ministry building, activists said.

Nouh al-Wafi said two protesters were killed and four wounded in the volatile city of Taiz, and another protester was killed in the city of Damar after demonstrators calling for the ouster of longtime President Ali Abdullah Saleh clashed with police.

After the second protester was killed in Taiz, demonstrators stormed the police station where the gunfire came from, seized a policeman whom they accused of shooting, and handed him over to the prosecutor's office, said another activist, Ghazi al-Samai.

He also said protesters were setting fire to tires in several streets in Taiz and took control of three government buildings, including the Oil Ministry.

In Sanaa, the capital, protesters started a second sit-in on one of the city's main streets, adding to the already large permanent sit-in on Change Square near Sanaa University, al-Wafi said.

Tawakul Karman, a senior member of the main opposition party, Islah, said plans were developing for protest marches on the presidential palace in Sanaa, and other government buildings elsewhere, to press Saleh to step down.

In Aden, another southern city, demonstrators also set fire to tires in the streets as the city was paralyzed by a civil disobedience called by the opposition. Similar demonstrations took place in Hadramawt and Hodeida.

Yemen has been reeling from nearly three months of protests demanding Saleh's ouster. In office for over three decades, Saleh has intensified his crackdown on the protests and refused a regional mediation offer.

More than 140 people have reportedly been killed in the government crackdown on the escalating protests.