By Mohamed Sudam and Khaled Oweis
SANAA/DAMASCUS (Reuters) - Popular unrest swelled into a crisis for one Arab autocrat and began to rattle another long seen as immune on Monday as leading figures parted ways with Yemen's president and street protests spread in Syria.
Top generals, ambassadors and some tribes endorsed the goals of Yemen's anti-government protesters in a glancing blow to President Ali Abdullah Saleh as he strove to withstand demands for his resignation after 32 years in power.
Saleh, an important U.S. and Saudi ally in the world's paramount oil-exporting region, has survived a civil war, tribal revolts and al Qaeda militant campaigns so far.
However, the defections of major officials appeared to pose the gravest threat yet to his tenure, although some important military allies remained loyal. ' The three-month-old tide of revolt against Arab rulers seen as repressive, corrupt and unaccountable for unemployment and poverty reached Syria on Friday and gained momentum on Monday as rallies for "freedom" spread in the south.
President Bashar al-Assad's security forces killed four civilians in demonstrations that erupted in the southern city of Deraa on Friday, uncorking the sharpest challenge yet to his 11-year-old rule. His Baath Party has dominated Syria since 1963.
In Libya, where peaceful protests grew into insurrection within weeks, Muammar Gaddafi's forces besieged the only major rebel city in the west and brought in human shields, residents of Misrata said, to ward off further Western air strikes meant to protect civilians.
The first French and British sorties, backed by U.S. missile salvoes, at the weekend halted the Gaddafi forces' advance on the rebels' eastern power base of Benghazi and pounded Libyan air defenses to enforce a U.N.-declared no-fly zone.
As Western forces prepared to shift from air strikes to air patrols, there was no sign of any gains on the ground by the loosely organized, poorly trained insurgents.
Nevertheless, a senior official with the rebel National Council, based in Benghazi, said the rebels aimed to capture the capital Tripoli and drive out Gaddafi. He welcomed the no-fly zone but again said no foreign ground forces were wanted.
International divisions over intervention in Libya flared into the open on Monday as leaders in Russia and China criticized the air strike policy which neither approved in the U.N. Security Council vote last week.
Russian Prime Minister Vladimir Putin compared the air strikes to "medieval crusades," eliciting a rare rebuke from his protege, President Dmitry Medvedev, while China's official media accused countries involved of inviting new upheaval in the Middle East. Turkey and Germany, both members of NATO, voiced further doubt and the Arab League was also divided.
BAHRAIN SAYS FOREIGN PLOT FOILED
In Bahrain, where the absolute Sunni Muslim monarchy has been grappling with a restive Shi'ite majority, said a foreign plot against the Gulf island state had been thwarted.
The head of the Gulf Cooperation Council said interference in Gulf state affairs by Iran, the Shi'ite giant across the water from Bahrain, would not be tolerated.
Confrontation between Bahraini Sunnis and Shi'ites calling for democracy has whipped up international tension in the region. About 1,000 Saudi soldiers and 500 police officers from the United Arab Emirates entered Bahrain last week to safeguard government facilities.
Kuwaiti naval vessels arrived in Bahrain on Monday to help protect its waters as part of a Gulf defense pact, Bahrain's state news agency said. Bahrain is the base for the U.S. Fifth Fleet, central to Washington's military power in the oil-rich region.
France became the first Western power on Monday to call publicly for Yemen's Saleh to stand down. Pan-Arab TV channel Al Arabiya quoted Saleh as saying most Yemenis were with him and he would remain steadfast.
A government official said the president had asked Saudi Foreign Minister Saud al-Faisal to mediate in the crisis.
Defense Minister Mohammad Nasser Ali said the army still backed Saleh, setting the stage for a possible standoff with those commanders who threw in their lot with tens of thousands of protesters who have taken to the street for weeks.
The latest defections seemed to be sparked by Yemen's bloody response to protests on Friday, when plainclothes snipers killed 52 protesters in Sanaa, prompting Saleh to sack his cabinet and declare a 30-day state of emergency.
In Syria, hundreds of people demonstrated against the government in the southern town of Jassem, activists said, but authorities did not use force to quell the latest protest.
"This is peaceful, peaceful. God, Syria, freedom!" chanted the protesters in Jassem, an agricultural town near Deraa.
Syrian Justice Minister Mohamad Ahmad Younis went to Deraa to try to calm emotions and open a dialogue with protesters.
The authorities appeared to adopt less heavy-handed tactics, opting not to intervene against protests demanding freedom and an end to graft and repression, but not the overthrow of Assad.
The Baath Party has banned opposition and enforced emergency laws since seizing power in a coup 48 years ago.
The Pentagon said on Monday Libya air strikes had hamstrung Gaddafi's might but signaled that the United States would keep the rebels at arm's length to avoid getting mired in a messy civil conflict amid war weariness over Afghanistan and Iraq.
General Carter Ham, U.S. commander leading a multinational coalition against Gaddafi, said three days of air and missile strikes had fashioned a broad no-fly zone but Western forces were not providing close air support for Libyan insurgents.
(Additional reporting by Maria Golovnina, Michael Georgy, Angus MacSwan and Mohammed Abbas in Libya, Dominic Evans in Beirut, Cynthia Johnston in Sanaa, David Brunnstrom in Brussels, John Irish in Paris, Adrian Croft in London, Missy Ryan and David Alexander in Washington; writing by Mark Heinrich; editing by Andrew Dobbie)