Yemeni President Ali Abdullah Saleh imposed a 30-day state of emergency on Friday after protest violence killed at least 42 and injured more than 300.

Mr. Saleh expressed "sorrow for what happened today" after government loyalists opened fire on opposition demonstrators in the capital, but insisted that security forces were not behind the killings.

"The national security council announces a state of emergency across Yemen, and a curfew is set upon armed people in all Yemeni provinces," said Saleh, who said armed protesters had been behind the gunfire. "And the security forces with the Army will take responsibility for stability."

The intensification of force used against demonstrators has some concerned that protesters will retaliate, threatening the possibility of a broad war that could engulf the country. Yemen is the second most heavily armed country in the world, behind the US.

Friday’s events mark the largest escalation of violence since Yemen’s protest movement began in earnest in mid-February, more than doubling the number of deaths throughout the country in a single day. Prior to the shooting, approximately 40 had been killed, according to Amnesty International. Witnesses said that children were among the hundreds wounded by gunfire.

Just after Friday prayers, men armed with semiautomatic weapons began firing on protesters from rooftops of buildings overlooking the area. Massive clouds of black smoke could be seen billowing from the edge of the demonstration area.

“As soon as we got up from prayer they started firing from the tops of multiple buildings in the area,” said Essam al-Maqtary, a Sanaa resident who was shot in the leg. “The baltageya [thugs] lit tires on fire so nobody could see exactly where they were and so they couldn’t be recorded on video.”

Approximately 100,000 demonstrators convened at the university on Friday, calling for the resignation of Yemen’s 32-year president. The number of demonstrators has increased steadily each week since the fall of former Egyptian president Hosni Mubarak on Feb. 11. Thousands have been camped out on the streets lining the university for weeks, but many Sanaa residents join the protest on Fridays for prayer services.

After the violence today, 21 young men killed amidst the melee were laid in a row inside a mosque that acts as a makeshift hospital, Qurans placed on their chests. Nearly all had been shot in the head or neck.

"We condemn these crimes," said Yassin Noman, president of Yemen's opposition coalition, factions of which had previously supported dialogue. "There is no longer any possibility of mutual understanding with this regime and he has no choice but to surrender authority to the people."

Protesters blame thugs

Official security forces, which had amassed at previous protests and used tear gas to disperse the crowds, were not visibly present at the scene of incident today – leading some to speculate that the violence had been condoned.

“Security was there,” asserted Majdy al-Assad, a Sanaa University student. “These thugs that killed people are the military in civilian clothes.”

As gunfire rang out, thousands of demonstrators stormed the area, fighting against a steady stream of volunteer medics carrying away the dead and wounded to the mosque.

“We must be united,” one called out as he ran towards the gunfire.

Armed tribesmen: 'We are stronger than the Army'

As violence has increased in frequency in recent weeks, the opposition crowd at the university has evolved in response. Thousands of tribesmen coming from Yemen’s mountainous regions to the north and east have joined calls for the fall of Mr. Saleh’s regime. They now appear to make up at least half of the opposition demonstrators.

Want to read more quality stories from our correspondents on the ground? Sign up for our daily World Editor's Picks newsletter.

Their presence has thus far acted as a balancing force to Yemen’s state security forces. Some of the tribes are known to be well armed and often engage in revenge killings. Until Friday, many believed that their participation in the protests would dissuade the government from allowing a major escalation in violence.

“We’re here to demonstrate peacefully with the people and call for human rights and democracy,” said Abdullah al-Imradi, a tribesman from al-Jawf who insisted that Friday's violence would not yet provoke a violent response from the tribes. “We don’t want this to become violent, but we will be prepared for it. We are stronger than the army.”