Published: Mar 18, 2011 8:27 PM EDT
Updated: Mar 18, 2011 7:25 PM EDT

SANAA, Yemen - As snipers hidden on rooftops fired methodically on Yemeni protesters Friday, police sealed off a key escape route with a wall of burning tires, turning the largest of a month of anti-government demonstrations into a killing field in which at least 46 people perished.

Many of the victims, who included children, were shot in the head and neck, their bodies left sprawled on the ground or carried off by other protesters desperately pressing scarves to wounds to try to stop the bleeding.

The dramatic escalation in President Ali Abdullah Saleh's crackdown suggested he was growing more fearful that the unprecedented street protests set off by the region's unrest could unravel his 32-year grip on power in the volatile and impoverished nation.

The United States, which supports Saleh's government with hundreds of millions of dollars in military aid to battle one of al-Qaida's most active franchises, condemned the violence. President Barack Obama said those responsible must be held accountable, and Saleh should honor his pledge to allow peaceful demonstrations.

Instead, Saleh declared a nationwide state of emergency that formally gave his security forces a freer hand to confront demonstrators. It will remain in force for 30 days and bars citizens from carrying weapons.

The protest in the capital, Sanaa, drew tens of thousands, the largest crowd yet in the popular uprising that first gathered pace a day after President Hosni Mubarak was toppled in Egypt in a stunning 18-day revolt over similar grievances of corruption, poverty and repression. Yemen's protesters are demanding Saleh's ouster, rejecting offers to discuss a unity government.

A military helicopter flew low over the square in Sanaa just as protesters were arriving after the main Muslim prayer services of the week.

A short while later, gunfire rang out from rooftops and houses, sending the crowd into a panic. Dozens were hit and crumpled to the ground. One man ran for help cradling a young boy shot in the head.

Police used burning tires and gasoline to make a wall of fire that blocked demonstrators from fleeing down a main road leading to sensitive locations, including the president's residence.

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

Enraged protesters stormed several buildings where the snipers had taken position, dragging out 10 people, including paid thugs, who they said would be handed over to judicial authorities.

Witnesses said the snipers wore the beige uniforms of Yemen's elite forces and that others were plainclothes security officers. President Saleh denied at a press conference that government forces were involved and said he ordered the formation of a committee to investigate.

A Yemeni photojournalist, Jamal al-Sharaabi, was among the dead, medical officials said. He is the first journalist killed in the unrest.

The bloodshed failed to dislodge the protesters, who stood their ground around the square, hurling stones at security troops and braving live fire and tear gas.

Throughout the weeks of unrest, security forces and pro-government thugs have used live fire, rubber bullets, tear gas, sticks, knives and rocks against the protesters, who have only grown in number in Sanaa and in many other cities around the nation. The protesters say they won't go until Saleh does.

"They want to scare and terrorize us. They want to drag us into a cycle of violence - to make the revolution meaningless," said Jamal Anaam, a 40-year-old activist camping out in the plaza that the protesters call "Taghyir Square" - Arabic for "Change."

"They want to repeat the Libyan experiment, but we refuse to be dragged into violence no matter what the price," he said.

Friday's violence showed the government of Saleh and his family is increasingly worried about losing power, said Gregory Johnsen, an expert on Yemen at Princeton University.

"He has been in power for more than three decades and he's falling back on what he knows best, which is increasingly violent methods."

The tactic is unlikely to work, he predicted.

"Yemen does not have a population that's easily cowed, so I don't think they will be put out by fear of death. ... It's a heavily armed country, many of the people there are quite confident and capable of putting security into their own hands," he said.

In the latest defection by a political ally of the president, Nabil al-Faqih, the Yemeni tourism minister, resigned Friday from his Cabinet position and from the ruling party to protest the killings.

"This is the least I can do," he said. Al-Faqih is the second minister to quite and the latest in a string of politicians to resign from Saleh's Congress Party.

Saleh and his weak government have faced down many serious challenges, often forging tricky alliances with restive tribes to delicately extend power beyond the capital. Most recently, he has battled an on-and-off seven-year armed rebellion in the north, a secessionist movement in the south, and an al-Qaida offshoot attacking his forces and targets beyond the country.

Of biggest concern to the U.S. is the latter. Al-Qaida in the Arabian Peninsula, which formed in January 2009, has moved beyond regional aims and attacked the West, including sending a suicide bomber who came terrifyingly close to blowing up a U.S.-bound airliner with a bomb sewn into his underwear. The device failed to detonate properly.

Yemen is also home to the U.S.-born radical cleric Anwar al-Awlaki, believed to have offered inspiration to those attacking the U.S.

American military aid to Yemen is to reach $250 million this year. That cooperation has also riled opponents of Saleh's rule even before the current unrest.

In his statement on Yemen Friday, Obama said, "The United States stands for a set of universal rights, including the freedom of expression and assembly, as well as political change that meets the aspirations of the Yemeni people."

He said it was now more important than ever for all sides in Yemen to work toward "a peaceful, orderly and democratic path to a stronger and more prosperous nation."

U.N. Secretary-General Ban Ki-moon also condemned Friday's violence and was "deeply troubled," said his spokesman, Martin Nesirky. He "reiterates his call for utmost restraint and reminds the government of Yemen that it has an obligation to protect civilians."

Doctors at a makeshift field hospital near the protest camp at Sanaa University confirmed at least 46 dead, three of them children. They spoke on condition of anonymity because they were not authorized to speak to the media.

Medical officials and witnesses say hundreds were wounded.

Interior Minister Gen. Mouthar al-Masri, who is in charge of internal security forces, put the number of dead at 25 and the injured at 200.

Amnesty International, which deplored Friday's bloodshed and demanded an investigation, said that 80 people have been killed since the start of the unrest in Yemen in early February.

--- Karam reported from Cairo.

(Copyright 2011 by The Associated Press. All Rights Reserved.)