Yemen's main opposition coalition says it will accept a transfer of power to the vice-president, after President Ali Abdullah Saleh left for Saudi Arabia.
Otherwise, the opposition Joint Meeting Parties said, they would seek to form a transitional government.
Vice-President Abd-Rabbu Mansour Hadi stepped in when Mr Saleh departed on Saturday for medical treatment.
Mr Hadi said Mr Saleh, who was wounded last week in an attack on his compound in Sanaa, would return within days.
Mr Saleh, who has ruled since 1978, has refused to leave office despite protests and a tribal uprising which has brought the country to the brink of civil war.
Meanwhile, at least three people died on Monday in the capital Sanaa in clashes with security forces, according to the Ahmar tribe.
"The opposition supports the complete transfer of power to the vice-president," Sultan el-Atwani, a leading figure in the Joint Meeting Parties, told Reuters news agency.
"In the case this falls through, the opposition and the youth of the revolution have alternative options, meaning a transitional council."
Mr Saleh had refused three times to sign a deal brokered by Gulf countries under which he would have handed power to Mr Hadi in exchange for immunity from prosecution.
Saudi Arabia said it would continue to encourage parties to accept the deal, according to the country's state media.
Thousands of people have been celebrating Mr Saleh's departure.
However, there are mounting fears for the stability of the country, which is home to an al-Qaeda wing and riven with tribal and political divisions.
Chest and neck surgery
Saudi officials said Mr Saleh had undergone surgery to remove shrapnel from his chest.
There were reports he would remain in Saudi Arabia for two weeks; one week to recover and another for meetings, but it was not known what he planned to do after that.
Mr Hadi said the president was recovering well and "would return to the homeland in the coming days", according to the state news agency Saba.
He added that he had spoken with Mr Saleh on Sunday evening and Monday morning by phone, the news agency said.
Deputy Information Minister Abdu al-Janadi said that, if Mr Saleh left power, it would be done "in a constitutional way".
Meanwhile, thousands of people loyal to the president have dedicated Monday as a day of fasting to express support for the leader.
Even if President Saleh wants to return, it is unlikely Saudi Arabia will allow him, BBC Middle East correspondent Jon Leyne says.
Acting President Hadi has already met US ambassador Gerald Michael Feierstein, state news agency Saba reported, to discuss "the importance of co-operation with the [opposition] Common Forum" alliance.
He may have little real power, however, with Mr Saleh's son and other relatives in charge of key units of the security forces.
Friday's attack on Mr Saleh came after days of street battles in Sanaa between government forces and fighters loyal to Sheikh Sadiq al-Ahmar, head of the powerful Hashid tribal federation.
That power struggle overlaid widespread street protests that began earlier in the year, inspired by the uprisings in Tunisia and Egypt, calling for democratic reforms and an end to Mr Saleh's rule.
Some analysts say Saudi Arabia will now use his presence in the country to pressure him to sign the agreement.