SAN'A, Yemen – Yemeni security forces on Thursday opened fire on thousands of protesters in the south chanting anti-government slogans, killing 12 and wounding scores of others, medics and eyewitnesses said.

Yemen's north and south were separate countries until they united in 1990, only to dissolve into a monthslong civil war four years later when the south tried unsuccessfully to secede.

Demonstrations by former army members in southern Yemen demanding political reforms have been occurring regularly since August 2007, underlying the continuing tensions in the south.

Eyewitnesses said hundreds of security forces opened fire on some 5,000 protesters in Abyan's provincial capital of Zinjibar to disperse them. The eyewitnesses spoke on condition of anonymity for fear of government reprisal.

A doctor at al-Razi government hospital in Zinjibar said ambulances rushed to the scene and brought back 10 dead civilians and at least 12 injured police. A doctor at Aden's May 28th hospital said he received 8 critically injured civilians, two of whom later died.

The doctors spoke on condition of anonymity because they were not authorized to talk to the media.

"Government forces have committed a massacre against unarmed civilians, firing live bullets and killing 10 people," said Ali Dehmes, an opposition member in the south, apparently referring to the protesters who died at the scene.

The governor of Abyan province, Ahmed al-Maysari, denied security forces fired on the protesters and said 8 civilians were killed when the demonstrators started shooting.

Some 60,000 southern servicemen were discharged from the army after the civil war, and many of them fled abroad. Most have since returned, attracted by amnesty and promises they would be allowed to re-enlist. But many have not been allowed back into the military, which is dominated by northerners.

Some people participating in Thursday's demonstration called for a revolution in the south, while others complained about deteriorating services, including scarce water and frequent power outages.

One eyewitness said that the special government plainclothes militia known as Unity Supporters used batons to beat up protesters and drag them across the ground into waiting police trucks.

The provincial deputy governor, Saleh el-Shamsi, said that the police now have the situation under control.