Yemeni security forces and pro-government loyalists opened fire on protesters marching in two cities on Monday, killing at least 12 and wounding scores, according to witnesses.

The violence was the deadliest attack on demonstrators, inspired by the populist rebellions of Egypt and Tunisia, since March 18, when snipers loyal to Yemeni President Ali Abdullah Saleh shot dead at least 52 protesters in the capital Sanaa. That event triggered wide scale defections of Saleh’s top allies from the military, tribes, and government.

In the southern city of Taiz, according to witnesses and televised images, police clutching guns, tear gas canisters and batons targeted unarmed protesters marching toward a provincial government building, killing 12 and wounding dozens.

Many of the victims were seriously injured and medical officials expected the death toll to rise. The injured were taken to a makeshift hospital, and images on local television showed men, who were apparently tear-gassed, on the floor being treated by nurses.

Twelve people died from wounds received on Monday, while two other succumbed to injuries from violence on Sunday, said a medical source at a makeshift hospital erected near a square that is focal point of the uprising. More than a dozen were in critical condition, the source added.

“The first four of the protesters who were killed were shot by snipers at the governor’s office,” said Yaser Alnusari, a medic in Taiz said in a phone interview. “The protesters were in tens of thousands and were protesting on most of the main streets in Taiz. They were condemning the violent actions that took place against them yesterday.”

Medical workers struggled to help the wounded. “The field hospital cannot give health care to all the injured, and is coordinating with hospitals outside Taiz to help the injured,” said Sadiq Alshuka, the head of the medical team.

According to a senior official in Taiz, the security forces were forced to open fire after thousands of protesters refused to disperse after they surrounded the governor’s office.

“More than 20 governmental tanks and armored vehicles are now at the government building trying to control the situation,” the official said.

“The regime has surprised us with this extent of killing. I don’t think the people will do anything other than come out with bare chests to drain the government of all its ammunition,” parliament member Mohammed Muqbil al-Hamiri told Al-Jazeera television.

As in other parts of the nation, people in Taiz have launched a civil disobedience campaign. Government offices and most shops were closed on Monday, witnesses said.

In the Red Sea port of Hudaida, security forces wearing both uniforms and plain clothes fired live rounds and tear gas at hundreds of demonstrators marching on a presidential palace, wounding at least 250, Reuters reported.

“They suddenly gathered around the province’s administrative building and headed to the presidential palace, but police stopped them by firing gunshots in the air and using tear gas. I saw a lot of plainclothes police attack them too,” a witness in Hudaida told Reuters by phone.

The attacks come as the government has lost control over several governorates while opposition groups have called for general strikes and civil disobedience campaigns in the south.