Armed with guns, knives and swords, supporters of Yemen's leader trapped U.S., European and Arab ambassadors at a diplomatic mission in new turmoil that swept across the capital Sunday as the president refused to sign an agreement calling for him to step down in 30 days.

Security forces broke up the crowd after several hours of letting them besiege the embassy. But President Ali Abdullah Saleh's balking at the U.S.-backed deal threatened to wreck hopes for a peaceful resolution to the chaos that has consumed this key Arab nation, where hundreds of thousands have protested for three months, defying a bloody crackdown, to demand his ouster.

If the mediation collapses, many fear further deterioration of the political situation, including an escalation of armed conflict between Saleh's loyalists and military units that have joined the opposition.

At nightfall Sunday, tensions were high in Sanaa, the capital. Pro-government gunmen and soldiers locked down main streets around the capital with roadblocks, while tens of thousands of anti-Saleh demonstrators were massed at their protest camp in a central Sanaa square, worried that a new crackdown could ensue.

Saleh refused twice before to sign the agreement. But this weekend it had appeared he was finally relenting, under intense pressure from his allies, the United States and Gulf Arab countries that mediated the accord. The opposition parties signed the accord on Saturday, and the Yemeni president grudgingly promised he would sign the following day.

Instead, the mercurial leader showed his determination to cling to the power he has held for 32 years, despite increasing isolation. His regime unleashed hundreds of armed loyalists into the streets of Sanaa in an apparently orchestrated campaign to demand he not step down. They demonstrated outside several embassies and blocked the road in front of the presidential palace, chanting, "We will not permit the president's ouster."

The American ambassador, along with the ambassadors of Britain, the European Union, Saudi Arabia and other Gulf Arab nations, had gathered at the United Arab Emirates Embassy, waiting to be taken to the presidential palace for the expected signing at noon.

They were besieged by a mob of hundreds of Saleh loyalists, wielding knives, swords and automatic rifles, diplomats inside the mission and witnesses said. The crowd blocked the entrances to the mission, trapping the diplomats inside. At one point, they attacked a convoy bringing the chief mediator of the accord — Abdullatif bin Rashid al-Zayani, head of the Gulf Cooperation Council. They pounded on the car as it entered the compound, witnesses said.

Hours later, Yemeni military helicopters ferried the U.S. ambassador and al-Zayani to the presidential palace to witness the signing, witnesses said. Yemeni state TV showed several top figures from Saleh's ruling party signing the accord as the president and American ambassador, Gerald M. Feierstein, watched. But Saleh himself refused.

Saleh said afterward he would not do so unless opposition leaders come to the palace and sign it as well in public, not "behind closed doors."

"If they don't comply, they are dragging us to a civil war, and they will have to hold responsibility for the bloodshed in the past and the blood which will be spilled later on because of their stupidity," Saleh warned in an address on state TV.

Afterward, the U.S. ambassador returned to the Emirati mission. Diplomats said that by evening, security forces — who had done nothing to stop the siege throughout the day — dispersed the militiamen and the ambassadors left. U.S. Embassy officials could not be reached for comment.

The opposition appeared to dismiss Saleh's demands that they participate in a public signing. Yemeni opposition official Abdul-Malak al-Mukhlafi said the mediators from the Gulf Cooperation Council — a grouping of six Gulf Arab nations — had set down in a three-page document the details of how the deal was to have been signed and that any change in that process was considered a breach of the agreement.

"This regime is taking the world and its people lightly," he told The Associated Press. "We ask the international and regional community to pressure the regime and force it to respect the will of the people, and to impose sanctions that will make it respond to the people's demands."

U.S. Secretary of State Hillary Rodham Clinton said the Obama administration was "deeply disappointed" by Saleh's refusal to sign an agreement to step down. She said he "is turning his back on his commitments and disregarding the legitimate aspirations of the Yemeni people."

The secretary also said she was "outraged" at Saleh's supporters surrounding the embassy in Sanaa.

In the Saudi capital Riyadh, five foreign ministers from the Gulf council held an emergency meeting Sunday to discuss what happened. A Saudi diplomat said there is "anger" over Saleh's position. He spoke on condition of anonymity because he was not authorized to talk to the media.

The ministers decided to suspend the initiative "because conditions were not ripe," according to a statement from the meeting. Still, they urged Saleh to sign in the "soonest possible time," he said.

But Saleh has proven a master of slipping from the grasp of U.S. and Arab leaders trying to pin him down on an agreement. Twice in the recent weeks, he has balked at the last minute. Once he insisted the composition of the opposition delegation be changed. When the opposition complied, he pulled another demand, asking that the protesters on the streets be dispersed first.

Saleh has managed to cling to power despite near daily protests by hundreds of thousands of Yemenis fed up with corruption and poverty. Like other anti-government movements sweeping the Arab world, they took inspiration from the popular uprisings in Tunisia and Egypt.

The president has swung between offering concessions, taking them back and executing a violent crackdown that has killed more than 150 people, according to the opposition.

Saleh has been hit by mass defections by ruling party members, lawmakers, Cabinet ministers and senior diplomats, as well as leaders of his own tribe and top army commanders, who deployed their tanks in the streets of Sanaa to protect protesters.

But Saleh has kept the loyalty of Yemen's most highly trained and best-equipped military units, which are led by close family members.

Saleh has warned several times that without him, al-Qaida would take control of the country. The United States has supported Saleh with financial aid and military equipment to fight the terror network. Al-Qaida in the Arabian Peninsula has an estimated 300 fighters in Yemen and has been behind several nearly successful attacks on U.S. targets, including the December 2009 attempt to bomb a Detroit-bound U.S. passenger jet.

The accord calls for Saleh to step down in 30 days and hand power to his vice president. It also calls for presidental elections and the formation of a national unity government. In return, Saleh would get immunity from any prosecution.

While the organized opposition parties have agreed, many of the different groups protesting his rule reject the deal, saying it falls short of their demands for an immediate departure and the dismantling of the regime.

Hundreds of thousands poured into a central square Sunday that has become the center of opposition protests, waving Yemeni flags and shouting rejection of the deal. They held banners that read: "Now, now Ali, down with the president!"

Women mingled with men, unlike in previous protests when female protesters stood on the edge of the square segregated from men. Children had their faces painted with Yemeni flags, while youths carried pictures of slain protesters. Young men and women held a 6-foot-long (2-meter) Yemeni flag.

"This initiative is only meant to save Ali not Yemen," declared Tawakul Karman, a protest leader and senior member of the opposition Islamic fundamentalist Islah Party. "We are going to continue our revolution until the end. Like Tunisia and Egypt, we will go against the opposition if they form a government while Saleh is still in power."


Associated Press Writer Maggie Michael in Cairo contributed to this report.