A day after 14 protesters were killed across Yemen, the Joint Forum condemned what it termed "a savage massacre committed by the authorities and the militia of the ruling family against peaceful demonstrators."
The US embassy in Sanaa said the upsurge in violence on the eve of a power transfer deal was "especially disturbing" and urged all sides to act with restraint.
It said Yemenis could demonstrate their support for a peaceful transition by avoiding all "provocative demonstrations, marches, and speeches in the coming days."
"We also urge government security forces to refrain from using violence against demonstrators," the embassy said in a statement.
Sanaa was the scene of the deadliest violence on Wednesday as troops opened fire to break up protests, killing 13 people and wounding more than 130, medical officials said. Another person was killed in southern Yemen.
Some protesters said they were attacked with daggers, which are traditionally worn in Yemen, while regime officials said "tens of supporters of the government" were also wounded.
"This massacre shows that the regime is determined to continue with the bloodshed and defeat the agreement" initiated by the Gulf Cooperation Council (GCC), said the Forum, a coalition of opposition parties.
It appealed to Gulf countries to do more to "protect peaceful demonstrators" rather than "provide protection to the killings of the regime."
The Forum warned that it would be difficult for them to go ahead with the planned signing of the transition agreement in Riyadh in coming days unless the crackdown was ended.
Amnesty International said the plan proposed by Yemen's oil-rich Arab neighbours in the Gulf appeared to provide blanket immunity to Saleh as well as his regime.
"President Ali Abdullah Saleh must not be allowed to evade accountability for the long catalogue of human rights crimes committed under his rule," said Malcolm Smart, Amnesty's director for the Middle East and North Africa.
He said Saleh and those around him should be held accountable for the arbitrary arrests, torture and unlawful killings that have been committed.
"He must not be awarded a ''get-out-of-jail'' card to walk free from any question of investigation or justice for what has been done under his authority," Amnesty said in a statement.