updated 3/22/2011 1:25:35 PM ET 2011-03-22T17:25:35

Yemen's embattled U.S.-backed president pledged to step down more than a year early but refused to immediately resign on Tuesday, infuriating tens of thousands of demonstrators demanding his ouster.

The opposition said it would not accept President Ali Abdullah Saleh's offer to resign by year's end in response to nationwide anti-government protests, which have swelled dramatically since security forces opened fatally shot more than 40 demonstrators on Friday.

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"The president's statements are just another political maneuver," said chief opposition spokesman Mohammed al-Sabri. "What was acceptable yesterday is not acceptable for us today."

The standoff pushed Yemen closer to open confrontation between the two sides, fueling Western fears that Saleh's 32-year-old regime could be replaced by chaos, or a leadership less likely to cooperate with U.S. military operations against the local branch of al-Qaida.

Slideshow: Political unrest in Yemen (on this page)

Anger at Friday's shootings splintered Saleh's remaining support among the country's most powerful institutions, and influential clerics, tribal leaders and military commanders all began calling for his departure. Some of the country's most senior army officials declared their loyalty to the opposition on Monday.

Saleh responded with a concession, pledging in a meeting with senior officials, military commanders and tribal leaders on Monday evening that he would to step down by the end of the year. He tried to placate demonstrators by promising this month not to run again, or let his son replace him, when his term ends in September 2013.

A presidential spokesman, Ahmed al-Sufi, said Salah also pledged not to hand power to the military.

The president hardened his position on Tuesday, saying the defection of commanders including his chief military adviser, Maj. Gen. Ali Mohsen al-Ahmar, was a "mutiny and a coup against constitutional legitimacy."

"Any dissent within the military institution will negatively affect the whole nation," Saleh said in a nationally televised warning to a meeting of Yemen's Supreme Council of the Armed Forces. "The nation is far greater than the ambition of individuals who want to seize power."

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Protesters massed by the tens of thousands Tuesday afternoon in the downtown Sanaa plaza they have dubbed "Taghyeer," or "Change" square. Crowds ululated, chanted and painted each other's faces in the red, white and black colors of the national flag. Conservative tribesmen bought their wives to the protest, and the women bought their children, all basking in a carnival atmosphere.

"The revolution has crossed its most difficult period," said activist Bashir al-Sid, smiling. "All that remains is the easy part."

Demonstrators began demanding Saleh's ouster more than a month ago, inspired by the wave of people power sweeping through the Middle East. His troops and loyalists have killed more than 80 demonstrators throughout Yemeni cities, according to an Associated Press tally of eyewitness, opposition and official accounts.

Al-Ahmar's defection was followed by a flurry of resignations by army commanders, ambassadors, lawmakers and provincial governors.

Al-Ahmar, commander of the army's powerful 1st Armored Division, deployed tanks and armored vehicles at the Defense Ministry, the TV building, the Central Bank and Taghyeer square, the demonstrator's epicenter.

In response, the Republican Guards, an elite force led by one of Saleh's sons, deployed troops backed by armor outside the presidential palace on the capital's southern outskirts.

Calling Al-Ahmar's defection "a turning point," Edmund J. Hull, U.S. ambassador to Yemen from 2001 to 2004, said it showed "the military overall ... no longer ties its fate to that of the president."

"I'd say he's going sooner rather than later," Hull said.

The Qatar-based Al-Jazeera network said its Yemen offices were broken into late Monday by 20 armed men who used a mechanical drill to open the door before seizing its main satellite uplink equipment, computers, TV cameras and closed-circuit surveillance cameras.

Slideshow: Yemen in the spotlight (on this page)

Jazeera said it had asked the Yemeni interior ministry three days ago to protect the bureau and its staff the network's journalists were threatened. The ministry did not respond, the network said.

In a sign of the Obama administration's growing alarm over the regime's crackdown on demonstrators, State Department spokesman Mark Toner called for "a cessation of all violence against demonstrators," saying Saleh should "take the necessary steps to promote a meaningful dialogue that addresses the concerns of his people."

The 65-year-old president and his government have faced down many serious challenges in the past, often forging fragile alliances with restive tribes to extend power beyond the capital. Most recently, he has battled a seven-year armed rebellion in the north, a secessionist movement in the south and an al-Qaida offshoot that is of great concern to the U.S.

Al-Qaida in the Arabian Peninsula, formed in 2009, has moved beyond regional aims and attacked the West, including sending a suicide bomber who tried to down a Detroit-bound airliner on Christmas Day with a bomb sewn into his underwear. The device failed to detonate properly.

Yemen is also home to U.S.-born radical cleric Anwar al-Awlaki, who is believed to have offered inspiration to those attacking the U.S., including Army Maj. Nidal Hasan, who is accused of killing 13 people and wounding dozens in a 2009 shootout at Fort Hood, Texas.

Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Video: Top Yemen commanders switch sides, join protesters

Photos: July

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  1. A man prepares the grave of Hassan al-Hora during his funeral at a cemetery in Sanaa, July 19. Fighting between government forces and opposition supporters erupted in Yemen's capital Sanaa on Monday, killing six people, among them al-Hora, opposition sources said. (Suhaib Salem / Reuters) Back to slideshow navigation
  2. Anti-government protesters shout slogans during a demonstration to demand the ouster of Yemen's President Ali Abdullah Saleh in the southern city of Taiz, July 19. (Khaled Abdullah / Reuters) Back to slideshow navigation
  3. A girl has ''will not leave'' written on her face during a rally to support Yemen's President Ali Abdullah Saleh in Sanaa July 17. (Suhaib Salem / Reuters) Back to slideshow navigation
  4. Anti-government protestors shout slogans during a demonstration demanding the resignation of President Ali Abdullah Saleh, in Sanaa, Yemen, on Wednesday, July 13. (Mohammed Hamoud / AP) Back to slideshow navigation
  5. An anti-government protester writes slogans on a wall using his own blood during a rally to demand the ouster of President Ali Abdullah Saleh at Tagheer square in Sanaa on July 13. The words read "In my blood I protect Yemen." (Suhaib Salem / Reuters) Back to slideshow navigation
  6. Girls light candles as they attend a rally to demand the ouster of Yemen's President Ali Abdullah Saleh in the southern city of Taiz July 9. (Khaled Abdullah / Reuters) Back to slideshow navigation
  7. A Yemeni anti-government protester displays bullets allegedly fired by supporters of Yemeni President Ali Abdullah Saleh during a demonstration demanding Saleh’s ousting, in Sana'a, Yemen on July 8. (Yahya Arhab / EPA) Back to slideshow navigation
  8. Supporters of Yemeni President Ali Abdullah Saleh sit on stone pillars during a rally in support of President Saleh in Sana'a, Yemen, on July 8. Supporters of Yemeni President Ali Abdullah Saleh staged rallies around his vacant palace Friday after their leader's first TV appearance since being injured in a blast last month and leaving for treatment in Saudi Arabia. (Mohammed Al-Sayaghi / AP) Back to slideshow navigation
  9. A supporter of Saleh kisses his picture as his supporters celebrate in Sanaa on July 7 after he appeared on television for the first time since he was severely wounded in an assassination attempt. (Mohammed Huwais / AFP - Getty Images) Back to slideshow navigation
  10. Saleh delivers a speech from the Saudi capital, Riyadh, on July 7, making his first public appearance since he was wounded in an attack on his palace in Sanaa in June. (AFP - Getty Images) Back to slideshow navigation
  11. Anti-government protesters join their hands and shout slogans demanding an end to the 32-year regime of President Saleh, in Sanaa on July 6. (EPA) Back to slideshow navigation
  12. A police vehicle is set ablaze during clashes between armed followers of the opposition and police in the southern city of Taiz on July 6. (Khaled Abdullah / Reuters) Back to slideshow navigation
  13. A relative of victims of recent clashes talks to a member of the United Nations human rights investigation team, left, in Sanaa on July 5. The U.N. team arrived in Yemen last week to assess the situation in the country after months of unrest. (Suhaib Salem / Reuters) Back to slideshow navigation
  14. Women recite prayers during a rally to demand the ouster of President Saleh in the southern city of Taiz on July 1. Tens of thousands of Yemenis turned Friday prayers into rallies for and against Saleh, who is recovering from injuries sustained in an assassination attempt in June. (Khaled Abdullah / Reuters) Back to slideshow navigation
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  1. Image: Man prepares the grave of al-Hora during his funeral at a cemetery in Sanaa
    Suhaib Salem / Reuters
    Above: Slideshow (14) Political unrest in Yemen - July
  2. Image:
    Hani Mohammed / AP
    Slideshow (39) Political unrest in Yemen - June
  3. Image: Anti-government protests in Yemen
    Wadia Mohammed / EPA
    Slideshow (59) Political unrest in Yemen - May
  4. Image:
    Hani Mohammed / AP
    Slideshow (25) Political unrest in Yemen - April
  5. Image: Tens of thousands of Yemenis take to the
    AFP - Getty Images
    Slideshow (67) Political unrest in Yemen - Earlier photos
  6. YEMEN
    Karim Ben Khelifa
    Slideshow (20) Yemen in the spotlight

Timeline: Yemen turmoil

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