Yasir Saeed was the first casualty dragged into the mosque. Blood was still trickling from the gaping bullet wound in the back of the 32-year-old English teacher's head as doctors lowered his mangled body on to a blanket, muttering a short prayer before closing the lids of his eyes.

Gradually, his corpse was joined by others lined up in a row alongside him. One by one, miniature Qur'ans were placed on their chests as their blood soaked through the carpet beneath them.

The scene was desperate and chaotic. It followed the worst day of violence in Yemen since protests against president Ali Abdullah Saleh began in earnest over a month ago. At least 45 people were killed and hundreds of others wounded as security forces and plainclothes government loyalists opened fire on protesters trying to march through the capital, Sana'a.

Parliamentary opposition spokesman Mohammed al-Sabri accused the regime of a massacre and said: "These killings will not help keep Ali Abdullah Saleh in power." Saleh responded by calling for a state of emergency, saying this meant that ordinary citizens would not be able to carry weapons.

But it was not immediately clear if Saleh has the military power to impose such an order, with the Arabian peninsula nation deeply divided and racked by weeks of civil disturbance that have left well over 70 people dead. "What happened today was very regrettable, the death of our children," the president said. Last week he had ordered his security forces to ensure the safety of protesters.

Saleh went on to describe the victims as "martyrs of democracy" and accused those responsible of trying to undermine a peace initiative backed by Saudi Arabia.

Friday's protests had started peacefully. More than 100,000 Yemenis filled a mile-long stretch of road by Sana'a University for a midday prayer ceremony mourning the loss of seven protesters killed in clashes with riot police last weekend.

As the prayers came to an end, however, the sight of billowing black smoke from a burning car caught the attention of protesters who began surging en masse towards it. Witnesses say security forces fired six shots into the air before turning their weapons on those charging towards them. As violence flared, plainclothes men appeared on the roofs of nearby houses and began firing down on the demonstrators with Kalashnikovs.

Sana'a University, the scene of the bloodshed, is next door to the mosque where many of the dead and dying were taken, the deafening sound of the muezzin's call to prayer intermixed with the noise of gunfire echoing off the walls. Medics scrambled to reach the wounded as the wheels of decrepit ambulances, trying to escort them to a proper hospital, spun hopelessly in the mud.

Inside the mosque a throng of veiled women wailed with grief and tried to force their way past a line of students who had linked arms to ensure only medics and those in need of their help made it over the threshold. Meanwhile injured men, most in their early 20s, writhed in agony on shabby mattresses on the ground.

"The most common injuries were bullet wounds to the chest and the head, although some are suffering from exposure to teargas," said one doctor who did not wish to be named.

"They shot people in the back of the head as they were running away," said Mohammed al-Jamil, an Indian doctor treating the wounded with specks of blood on his hands and face.

"Whoever did this wanted these people to die," he added, tearing open a box of syringes.

Witnesses said children were among the dozens wounded by gunfire. "My brother is 12 years old, they shot him twice, once in the arm and once in the leg," shouted a young man through a crackling microphone to a roaring crowd of thousands outside the mosque.

"Saleh would rather shoot us all before stepping down."

Until now government forces have largely been using water cannon, rubber bullets and teargas to disperse anti-regime rallies, but live rounds were fired on Friday in what appeared be the beginning of an increasingly violent crackdown on nationwide protests.

Yemen, the youngest and poorest country in the Arab world neighbouring Saudi Arabia, has been hit by weeks of protests set in motion by uprisings in north Africa that toppled long-serving leaders in Tunisia and Egypt and spread to the Gulf states of Bahrain and Oman, as well as Saudi Arabia itself.

Saleh has maintained a firm grip on power for over three decades and has rejected calls to step down, saying he will only do so when his current term of office expires in 2013.

"We condemn these crimes against humanity," said Mohammed al-Qadhi, a prominent member of Yemen's ruling party who resigned last week and has since survived two assassination attempts. "Even if it's plainclothes men firing on the protesters it is still the government's responsibility to protect them."

The intensification of force used against demonstrators has led to concern that protesters will retaliate, threatening the possibility of a broad war that could engulf the country.

"In Yemen, violence is almost always met with more violence. If the regime does not stop these crackdowns immediately then we will soon find ourselves in the throngs of a bloody civil war," said Mohammed al-Faqih, professor of politics at Sana'a University.