SANAA, Yemen — A top military officer and at least 18 other senior commanders defected Monday to the opposition movement demanding the immediate ouster of Yemen's embattled president, depriving the U.S.-allied ruler of most of his fast-dwindling power base.

The looming collapse of President Ali Abdullah Saleh's regime throws into doubt the American campaign against a major al-Qaida wing that plotted attacks in the United States.

Monday's defections led to rival tanks being deployed in the streets of Yemen's capital, Sanaa, creating a potentially explosive situation and prompting Saleh's defense minister, Mohammed Nasser Ahmed, to announce the military remained loyal to the longtime leader.

The armed forces will counter any plots against the government, Ahmed declared on state television, following a meeting of the National Defense Council, which is led by Saleh and includes Ahmed, the prime minister and the intelligence chief.

The defection of Maj. Gen. Ali Mohsen al-Ahmar, a longtime Saleh confidante and commander of the army's powerful 1st Armored Division, was seen by some as a turning point. It followed a major escalation in the regime's crackdown on demonstrators, when more than 40 people were killed in bloody clashes Friday.

Speaking in Paris, French Foreign Minister Alain Juppe called Saleh's resignation "unavoidable" and pledged "support to all those that fight for democracy."

Tanks, armored vehicles and soldiers directed by al-Ahmar fanned out around the Sanaa square that has become the epicenter of the opposition movement, moving in for the first time to protect demonstrators.

Al-Ahmar also sent tanks to the state television building, the Central Bank and the Defense Ministry. In response, at least a dozen tanks and armored personnel carriers belonging to the Republican Guards, an elite force led by Saleh's son and one-time heir apparent, Ahmed, were deployed outside the presidential palace on Sanaa's southern outskirts.

The deployment of al-Ahmar's troops in Sanaa was greeted by wild jubilation from protesters, many of whom posed with soldiers for photographs, greeted them with military style salutes or offered them roses.

Edmund J. Hull, U.S. ambassador to Yemen from 2001 to 2004, called Al-Ahmar's defection "a turning point along with the killings last Friday. It indicates that 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 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 that year 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.

Saleh has been a key, though not entirely reliable, U.S. ally in the fight against al-Qaida, frustrating his Washington backers with the delicate balancing act he has undertaken to maintain the goodwill of powerful tribes providing refuge to operatives from the terror network.

He has also earned a reputation for milking the "al-Qaida card," demanding millions of dollars in military aid that he has used to bolster the capabilities of units loyal to him rather than take on al-Qaida.

A Saleh successor would not be much different since Yemen's complex tribal system would stay intact after he is gone.

Al-Ahmar and two other senior army officers who defected Monday belong to Saleh's Hashid tribe and a tribal leader said it was rallying behind al-Ahmar as a possible replacement for Saleh, eager to keep the president's job for one of its own.

The leader spoke on condition of anonymity because of the sensitivity of the subject.

Monday's defections included at least 15 other top military figures. Among them were Sanaa's military commander, a former defense minister who served as a presidential adviser and a military police brigadier who is a member of the president's personal security detail.

A key tribal leader, Sadeq al-Ahmar, the chief of Saleh's Hashid tribe, also announced Monday that he and his supporters were joining the protest movement. Speaking to Al-Jazeera television from Sanaa, he said the death of scores of protesters on Friday made him decide to back the opposition after weeks of trying to mediate between Saleh and the protesters.

"The demands of the protesters are the demands of the Yemeni people," he said. "I can no longer fool myself, it is not the custom of men or tribes to do so."

Several top diplomats also said they were joining the opposition, including Yemen's ambassadors to Egypt, Syria, Jordan, Japan and the Arab League. Lawmakers, editors of state-owned newspapers, parliament's deputy speaker and the governor of the southern province of Aden also quit their jobs to join the opposition and urge Saleh to step down.

Meanwhile, in a sign of the deepening divisions in the armed forces, gunfire broke out late Monday between the central security force protecting the presidential compound in the port city of Mukalla and the Yemeni army outside, security officials said. The compound, where Saleh stays when he is in town, is about a half-mile (kilometer) from where hundreds of protesters have been camping out to call for is ouster.

Maj. Gen. al-Ahmar has been close to Saleh for most of the Yemeni president's 32 years in power. He has close associations with Islamist groups in Yemen that are likely to raise suspicions in the West about his willingness to effectively fight al-Qaida operatives active in the country.

He is a veteran of the 1994 civil war that saw Saleh's army suppress an attempt by southern Yemen to secede. Al-Ahmar also fought in recent years against Shiite rebels in northern Yemen.

His defection to the opposition was welcomed by protesters, but the warm reception may not guarantee him a political career in a post-Saleh Yemen given his close links to the president.

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"He comes from the very heart of Saleh's ruling dynasty," Yemeni analyst Mansour Haiel said of al-Ahmar, who has sometimes been seen as a rival to the president and his son, Ahmed.

"He could easily become the head of the next ruling dynasty."

In the southern port city of Aden, Muslim militants set fire to a jazz club and a bar, objecting to their serving alcohol, a security official said. Speaking on condition of anonymity because he was not authorized to release information to reporters, he said the men were part of an Islamist group taking advantage of the city's security void, as police were busy dealing with demonstrations.

Hendawi reported from Cairo.

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