© Agence France-Presse/Getty ImagesProtesters carry away a wounded youth after snipers opened fire at a demonstration on Friday
Photographs and amateur video footage have provided the first compelling evidence that professional snipers shot to kill when they opened fire on an anti-government demonstration in Yemen that left at least 52 protesters dead.

Image after image of the dead, men and boys, showed that those killed in the most violent day in the capital city Sana'a for 30 years had been systematically shot through the head and neck by gunmen positioned on city rooftops.

Yet even as the international community condemned Friday's violence, Ali Abdullah Saleh, Yemen's president of 32 years, remained unbowed as his security forces visited more bloodshed on protesters in the port city of Aden, a strategic British colony until 1963.

Police used live fire to disperse demonstrators calling for Mr Saleh's resignation. Local human rights groups said that at least four people died, while more than a dozen more were wounded.

Abandoning all pretence of defending his people's legitimate right to protest, a pledge that he had made just a week before, Mr Saleh deployed tanks on the streets of Sana'a after declaring a state of emergency.

But if he hoped to scare the ever-growing movement demanding his overthrow, Mr Saleh appeared to have misjudged the public mood.

On streets still stained with the blood of their comrades, protesters came in their tens of thousands to gather once more along the mile-long stretch of road outside Sana'a' University that has become the symbolic headquarters of the growing insurrection.

Just 24 hours earlier, many had stood defiant at the same spot as bullets flew around them.

Video footage released on Saturday suggested the callousness of a plot that seemed design to kill as many people as possible.

One clip shows smoke billowing from the southern end of the protesters' camp where unidentified men had erected a burning barricade of tyres to prevent the demonstrators, many of whom had been outside the university for days, from escaping.

Suddenly, there is a crackle of gunfire and scores of people duck involuntarily. But even as one man is felled, blood turning his white dishdasha robe red, others around him resume their anti-government slogans undeterred.

On the other side of what the demonstrators have begun to call "Taghyir" or "Change" Square, others stripped off their jackets and advanced towards the ever more relentless gunfire, pointing towards their chests as if in an invitation to shoot.

As the carnage continued, killing 52 and wounding hundreds more, victims were brought to a nearby mosque that had been turned into makeshift hospital.

Photographs showed the dead, identity cards and miniatures of the Koran laid on their corpses, lain in rows across the carpeted floor on an inner prayer room.

One young boy, barefoot and dressed in an Arsenal football club T-shirt, had been shot just above the eye. Another photograph showed a veiled woman cradling the body of her young son, his arms outstretched as if in supplication.

In one video, a man holding the body of his dead brother is shown making a tearful telephone call to his mother to tell her, in a faltering voice, that her son is dead.

Nearly all the bodies in the photographs had bullet wounds either in the forehead, neck or in the back of the head. There seems little doubt that this was the work of trained marksmen.

Yet President Saleh, while expressing his sorrow, claimed that the gunmen were either the demonstrators themselves or irate residents neighbouring the university who had grown tired of the noise of the protests - a claim denied by the residents themselves.

The opposition coalition at the forefront of the protests accused Mr Saleh, a key US ally against al-Qaeda, of perpetrating crimes against humanity.

Comment: Against al Qaeda? Aren't "al Qaeda" America's allies now?

Al-Qaeda probably responsible Syrian suicide bombings, US spy chief claims

"It is a massacre," said opposition spokesman Mohammed al-Sabri. "This is part of a criminal plan to kill off the protesters, and the president and his relatives are responsible for the bloodshed."

Much of the sniper fire emanated from a building allegedly owned by a regional governor close to President Saleh, further evidence, the opposition said, of the regime's involvement in the killings.

Video footage showed a masked man crouching behind a balustrade on the building's roof.

As the gunfire continued unabated, a group of protesters stormed the rooftop, braving gunfire to capture 10 of the snipers - seven of whom were said to have possessed government identification papers.

One of the suspected snipers was dragged into the streets, where he was beaten and clubbed by protesters, while a second was allegedly flung off the edge of the building.

Although 20 protesters had been killed in scattered incidents before Friday's violence, the scale of President Saleh's retribution proved too grotesque even for some of his longtime allies.

Three resigned in the aftermath, including one cabinet minister, a prominent ruling party figure who condemned the killings as "totally unacceptable" and Nasr Taha Mustafa, the high-profile head of the state news agency, a major source of regime propaganda.

"Nothing can justify the deaths of scores of youths whose only sin was to exercise the freedom guaranteed by Islam and the constitution to demand change," Mr Mustafa said.

Under the urging of Egypt's new military leadership, Mr Saleh had, before Friday, exercised a week of restraint after initially responding to protests that erupted last month with force.

But he appears to have changed course after hardliners in the Bahrain royal family reversed a policy of tolerance this week to shoot dead over a dozen protesters in this island kingdom before calling for and getting military intervention led by Saudi Arabia.

Backed by Saudi Arabia, and believing the West to be absorbed in implementing a no-fly zone in Libya, Mr Saleh apparently calculated that he could get away with a crackdown on the opposition protest movement, analysts said.

He may also have taken heart from only criticism for the United States, which did not repeat calls made in Egypt, Libya and Tunisia for regime change.

Seeing Mr Saleh as both a bulwark against the growing al Qaeda presence in Yemen and a safeguard against instability in a country plagued by two secessionist rebellions, the United States instead urged the protesters to be more "constructive" and engaged with the president.

In an effort to quell the unrest, Mr Saleh offered a raft of economic and political reforms as well as a pledge not to seek a further term at Yemen's next presidential election in 2013.

But Mr Saleh has promised not to contest elections in the past and few in the opposition trust him to honour his word this time.

With dozens of Mr Saleh's one-time allies deserting him, among them the head of a powerful tribal confederation to which the president belongs, the opposition senses that it now has the advantage.

"Sending tanks to the street is a sign that the regime is in a state of panic," said Mr al-Sabri.

"The widespread killing that took place, followed by the declaration of emergency law, demonstrate that the power of the people on the street has become greater than that of the government."

Although President Barack Obama condemned Friday's violence, Washington is also coming under pressure to take a more robust line with its ally and there have been calls for a suspension of US military aid to Yemen.

"Time and again, President Saleh promises he will stop attacks on peaceful protesters and yet the number of dead keeps rising," said Sarah Lee Whitson, the Middle East director for the New York-based Human Rights Watch.

"The United States should back up its words condemning the carnage with action and halt all military aid to Yemen."

Yet Washington remains uneasy about some of the high-profile figures among the protesters, in particular the prominent cleric Abdul Majeed al-Zindani, who has been termed a "Specially Designated Global Terrorist" by the US treasury department.

Mr Zindani, who is accused of once serving as a spiritual mentor to Osama bin Laden and has links with the radical Islamist cleric Anwar al-Awlaki, has attended the protests to call for the president's resignation and the creation of an Islamist state in Yemen.

Although Mr Zindani is influential, there is no evidence that his vision is shared by the bulk of the protesters, many of whom are lawyers and university professors from Yemen's middle class.

Hillary Clinton, the US secretary of state, has also publicly fretted about possible Iranian influence in the protests because Tehran has offered support to Houthi tribesmen waging a rebellion in the north of Yemen.

The Houthis, however, only represent a tiny fraction of the protests, analysts say.

While Washington's muted reaction has done little to endear it to the protester movement that could well form the next government in a key front line state in the war on terror, European diplomats are privately warning that Yemen could sink into a tribal war if Mr Saleh is not ousted soon.

"We are witnessing ever greater fragmentation within Yemen's all-important tribal structure," one western official said. "Allegiances are shifting quickly and Saleh's hold over the patronage system that keeps Yemen from collapsing is visibly slipping."

"It is not in the West's best interests to see this degenerate into a Libya-style conflict that would play into the hands of Islamist militants, which is why it would be better for Saleh to go sooner rather than later."