Tanks took up positions in key locations across Yemen's capital Sanaa including at the presidential palace, the central bank and the ministry of defence, but it was unclear what their orders were or who was in command.
In the first of a series of body blows to Saleh's authority, General Ali Mohsen al-Ahmar, commander of the Northwest Military District which includes Sanaa, announced he had joined the "revolution."
"The crisis is getting more complicated and it's pushing the country towards violence and civil war," the general said in a statement.
"According to what I'm feeling, and according to the feelings of my partner commanders and soldiers... I announce our support and our peaceful backing to the youth revolution.
"We are going to fulfil our duties in preserving security and stability."
Ahmar was followed by fellow generals Mohammed Ali Mohsen, the Eastern Military district chief, Nasser Ali Shuaybi in Hadramawt province and Faisal Rajab in the southern province of Lahij.
Dozens of officers of various ranks went to the tent city near Sanaa University, where demonstrators have kept vigil since February 21 in spite of a wave of attacks, and publicly pledged to support the revolution.
The deputy speaker of parliament, Himyar al-Ahmar, and the governor of the key southern province of Aden, Ahmed Qaatabi, also resigned in protest at the treatment of demonstrators.
Sadiq al-Ahmar, who leads the Hashid tribal federation, the largest in deeply tribal Yemen and a crucial source of Saleh's power, told Al-Jazeera it was time for the embattled president to make a "quiet exit."
The defections came a day after Saleh sacked his cabinet in a bid to placate opposition calls for sweeping reforms in the key US ally.
The regime has already lost the support of religious leaders and been weakened by the resignations of ministers, ambassadors and a host of ruling party MPs, but Saleh has refused to stand down until his term ends in 2013.
He said Monday the majority of the people were behind him.
His regime was internationally condemned after more than 50 people were killed when loyalist gunmen opened fire Friday on protesters in Sanaa's University Square, the centre of the pro-democracy movement.
The defection of top military officers to the opposition is likely to complicate Washington's support for Saleh, whom it sees as a pillar of stability in a volatile country and a partner in the war against Al-Qaeda.
UN Secretary General Ban Ki-moon, speaking in Cairo on Monday, strongly condemned the use of live ammunition against demonstrators in Yemen, and repeated international calls for dialogue and restraint.
In Syria thousands marched for the fourth straight day in the southern town of Daraa, after the funeral of a protester killed in the previous day's demonstration when security forces opened fire, a resident said.
"Just God, Syria and Freedom," and "Revolution, revolution" chanted the demonstrators, according to the resident who said security forces used tear gas and made several arrests in a bid to break up the protest.
The protesters, who have been inspired by regime-changing revolts in Tunisia and Egypt, are demanding "freedom" and an end to 48 years of emergency laws in Syria under President Bashar al-Assad and his father Hafez.
In Libya, Western forces launched new air strikes overnight, flattening a building in leader Muammar Gaddafi's Tripoli compound, while the Arab League reaffirmed its backing for a no-fly zone over the revolt-hit country authorised by the United Nations.
Gaddafi's troops retreated 100 kilometres (60 miles) from the insurgents' capital of Benghazi after fierce strafing by coalition aircraft destroyed much of their armour, but beat off a rebel advance on their new positions in the town of Ajdabiya.
Gaddafi's regime accused the coalition forces of violating a ceasefire which the military announced late Sunday, only to be accused by the United States promptly accused Tripoli of lying or of breaching the truce immediately.
Arab League Secretary General Amr Mussa said in Cairo on Monday he fully supported UN Resolution 1973, adding that his comments the previous day that the air strikes exceeded the UN mandate had been "misinterpreted."
Mussa said his earlier criticism had been motivated by concerns about civilians being caught up in the coalition strikes, as Arab governments did not want to see more deaths in Libya.
Ban, speaking at the Arab League headquarters in Cairo, said, "It is important that the international community speak with one voice" to implement the resolution."
But Russian Prime Minister Vladimir Putin Monday slammed the UN resolution -- which Moscow declined to veto at the Security Council -- as a "medieval call to crusade" and hit out at Washington for its readiness to resort to force.
In a fourth regional hotspot, Bahrain's King Hamad said the monarchy had foiled a "foreign plot" against Gulf countries, "prepared over a period of 30, maybe 20 years."
He was speaking to officers of a Saudi-led Gulf Cooperation Council force invited into Bahrain last week ahead of a crackdown on pro-democracy protests in the Shia-majority country that is ruled by a Sunni dynasty.
Tension has heightened between Bahrain and its Shia neighbour Iran, which has seen tit-for-tat expulsions of diplomats.