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Armed Yemeni government loyalists trapped Arab and Western diplomats inside an embassy on Sunday to block the signing of a political accord that would unseat President Ali Abdullah Saleh, witnesses said.

Saleh, a political survivor who has twice backed out of signing at the last minute, is under strong diplomatic pressure to seal the Gulf-brokered deal to end three months of protests against his rule.

The deal, if Saleh signs, would within a month make him the third Arab leader ousted by popular street protests this year. But a diplomat, speaking hours after Saleh was due to have signed, said chances of a signing were now looking slim.

"The issue of the ruling party signing is being complicated, and chances of finishing the deal are getting weak," a Gulf diplomat told Reuters in the Yemeni capital.

The deal would give Saleh immunity from prosecution, ensuring a dignified exit after nearly 33 years at the helm of the Arabian Peninsula state, located on a shipping lane through which three million barrels of oil pass every day.

Hundreds of armed Saleh loyalists rallied against the deal on Sunday, blocking main roads and preventing a Gulf mediator from heading to the presidential palace in Sanaa,.

Later they surrounded the United Arab Emirates embassy, witnesses said.

"We reject signing the Gulf initiative and the coup against legitimacy," some pro-Saleh demonstrators shouted from their cars over loudspeakers while others piled up stone barricades to block traffic.

Witnesses said the mediator, Gulf Cooperation Council Secretary-General Abdullatif al-Zayani, was stranded along with the U.S. and several European ambassadors in the embassy compound.

A U.S. embassy spokeswoman could not immediately be reached for comment.

The United States and Saudi Arabia, both targets of foiled attacks by al Qaeda's Yemen wing, are keen to end the Yemeni stalemate to avert deeper chaos that could give the militant network more room to thrive.

More than 170 protesters have been killed in a crackdown on demonstrations, part of the wave of uprisings across North Africa and the Middle East that swept aside the leaders of Tunisia and Egypt.

WANTS PALACE SIGNING

Yemeni opposition officials, representing a coalition that includes Islamists and leftists, signed the deal on Saturday after indications from Gulf mediators that Saleh would sign a day later. But the government side was adding new demands.

"The initiative must be signed at the palace in the presence of all sides," a ruling party official said.

The opposition said it had already signed the deal and would not comply.

The opposition is under pressure to avoid further compromises from youth-led street protesters -- including students and tribesmen -- who seek Saleh's immediate exit and who have vowed to continue daily rallies until Saleh quits.

Tens of thousands of protesters rallied peacefully on Sunday across Yemeni cities to keep up the pressure. They have threatened to step up their campaign by marching on government buildings, a tactic that led to more bloodshed this month when security forces opened fire to stop them.

Strikes have brought commerce to a halt in many cities and protests have created fuel shortages in a country already in turmoil -- Yemen faces revolts from Shi'ite rebels in the north and separatists in the south.

U.S. President Barack Obama said in a speech on U.S. policy in the Arab world on Thursday that Saleh needed to "follow through on his commitment to transfer power." European diplomats have also pressed both sides to agree on a deal.

Saleh, who has seen a wave of desertions including from within Yemen's military and political elite, called on Friday for an early presidential election he said was aimed at preventing bloodshed as protests raged on.

Saleh, a longtime ally of the West against al Qaeda, has warned Yemen's allies that al Qaeda could take over in a political and security vacuum after he steps down.

"We are for early presidential elections but we will not accept the president signing the initiative because it is a coup," loyalist activist Khaled al-Heimi said.