Tens of thousands of people pressed opposition demands on Wednesday for Yemeni President Ali Abdullah Saleh to resign, a standoff Gulf Arab ambassadors hope to help resolve at talks in neighbouring Saudi Arabia.
The ambassadors, members of the Gulf Cooperation Council, met opposition figures at the Saudi embassy in Sanaa to issue an invitation after Saleh accepted the GCC mediation on Tuesday.
"We only expect something good from our brothers. But it will take time before we can talk about the details," opposition parties' spokesman Mohammed Qahtan said after the meeting.
Faced with mass demonstrations demanding an end to his 32-year rule, Saleh is clinging to power in the poorest country in the Middle East, from which al Qaeda has planned attacks on the United States. He ignored a transition-of-power plan offered by the opposition on Saturday.
In Saudi Arabia, US Defence Secretary Robert Gates was due to discuss the unrest sweeping the region with King Abdullah, who has not seen mass protests but is concerned by Shiite complaints of discrimination and high youth unemployment.
Saleh has insisted for weeks he will leave once he has overseen parliamentary and presidential elections this year, rejecting an opposition proposal to allow the vice president, Abd-Rabbu Mansour Hadi, to become temporary head of state.
"The president will not leave his historic role early, before the transition of power... This issue is important," Saleh adviser Ahmed al-Sufi told Al Arabiya television.
Opposition sources have said talks stalled in recent weeks over Saleh's demands that he and his family should not face prosecution over corruption accusations by the opposition.
On Tuesday, an opposition source said security forces in the southern port city of Aden had detained six people for mobilising students to join a civil disobedience campaign that has kicked off in South Yemen in recent days, with shops, schools and some government offices closed for part of the day.
Tens of thousands resumed protests in Taiz, south of Sanaa, on Wednesday and security forces shot in the air to try to disperse them. There were no reports of casualties.
Tension has risen this week in a standoff that started in February when protesters began camping out outside Sanaa University.
On Monday, security forces and armed men in civilian clothes fired on protesters in Taiz and the Red Sea port of Hudaida, killing 21 people.
On Tuesday, security forces and armed men again attacked a crowd of tens of thousands of protesters in Taiz, residents said. Protesters responded by hurling rocks. Three people were killed in clashes in the capital Sanaa on Tuesday.
US changes tack
Washington has long seen Saleh as a pivotal ally in its fight against al Qaeda, which has used its Yemen base to stage attacks on Saudi Arabia and the United States. In return for billions of dollars in military aid, he has pledged to fight militants and allowed unpopular U.S. air strikes on their camps.
But on Monday, US officials said Washington was increasing pressure on Saleh to work towards a power transition plan.
On Tuesday, the Pentagon said the United States was calling for a negotiated transition in Yemen "as quickly as possible".
"Obviously the situation right now is a difficult one. The longer it festers, the more difficult it becomes," Pentagon Press Secretary Geoff Morrell said.
Some diplomats in Saudi Arabia have suggested Riyadh wants Ali Mohsen, the prominent general who turned against Saleh last month, to replace the president, though the general has said he is not interested in taking power.
Civil society opposition groups say Mohsen, 70, an Islamist, is tainted by his kinship and long-time association with the veteran ruler.
More than 100 people have been killed since anti-government protests began in Yemen, including the March 18 killings of 52 anti-government protesters by rooftop snipers in Sanaa.
That incident, which led Saleh to declare a state of emergency, prompted top Yemeni generals, ambassadors and some tribes to back the protesters, in a major blow to the president.
Frustration with Saleh's intransigence may push Yemenis, many of them heavily armed and no strangers to wars and
insurgencies, closer to a violent power struggle.