An anti-government protester waves the Yemen national flag as he sits on a lamp post during a rally outside Sanaa University, (Reuters).
Yemen opposition groups called on protesters to march on President Ali Abdullah Saleh's Sanaa palace on Friday to demand he step down, hoping to end a crisis that his allies abroad fear will benefit Islamic militants.
"Friday will be the 'Friday of the March Forward', with hundreds of thousands of people... We will arrive where you are and we will remove you," opposition spokesman Mohamed Qahtan told Al Jazeera on Wednesday, addressing the beleaguered Yemeni leader.
Seven weeks of street protests against Saleh's 32-year rule of the impoverished Arabian Peninsula state has raised alarm in Western capitals at the prospect of a country where al-Qaeda has entrenched itself.
Yemen borders the world's biggest oil exporter, Saudi Arabia, and major shipping routes. Al-Qaeda cells in Yemen have managed over the past two years to attempt brazen attacks outside Yemeni soil in Saudi Arabia and the United States.
Complaining of marginalisation, southerners have said they want to secede from Saleh's Yemen and northern Shiites have staged several rebellions against the perennial survivor, now in the biggest fight of his political life.
Long backed by Arab and Western countries as the strongman holding the fractious tribal country together, Saleh is raising the spectre of civil war and disintegration if he is forced out in what he says would be a coup.
A wave of defections among the ruling elite has reached senior military commanders, including General Ali Mohsen, commander of the northwest military zone and Saleh's kinsman from the al-Ahmar clan.
"Those who want to climb up to power through coups should know that this is out of the question. The homeland will not be stable, there will be a civil war, a bloody war.
They should carefully consider this," Saleh told army commanders on Tuesday.
Tension among rival military forces has led to violence.
Presidential guards - a force commanded by Saleh's son Ahmed - surrounded an air force battalion in the coastal city of Hudaida after its commander said he supported the protesters.
A presidential guard and a soldier died in clashes between the two forces in the southern coastal city of Mukalla late on Monday, medical sources said.
The United States, grappling with the diplomatic fallout of uprisings and uncertainty across the Arab world, voiced rare public alarm about the situation in Yemen and the possible fall of someone seen as an ally in the fight against al-Qaeda.
"We are obviously concerned about the instability in Yemen," U.S. Defense Secretary Robert Gates said. His chief concern was to avoid "diversion of attention" from opposing al-Qaeda there.
An aide to Saleh said he would leave office only after organising parliamentary polls and establishing democratic institutions, by January 2012 - a declaration the opposition promptly rejected. Saleh has already said he will not run when his term ends in 2013, an early verbal concession also rejected.
Yassin Noman, rotating head of Yemen's opposition coalition, offered Saleh the prospect of secure retirement if, like Egypt's former president Hosni Mubarak, he would go quietly, unlike Libyan leader Muammar Gaddafi.
"He shouldn't follow the style of Gaddafi by destroying the country and killing people," Noman said. "After this long term of governing, he should say: 'Thank you my people, I leave you peacefully'."
The defections, including generals, tribal leaders, diplomats and ministers, gained momentum after gunmen loyal to Saleh opened fire on demonstrators in the capital Sanaa on Friday, causing the deaths of 52 people.
The standoff is taking its toll on Yemen's fragile economy.
Liquefied natural gas producer Yemen LNG has told customers that unrest could lead to supply disruptions, leading stakeholder Total said.
Opponents complain that Yemen under Saleh has failed to meet the basic needs of the country's 23 million people. Unemployment is around 35 percent and 50 per cent for young people. Oil wealth is dwindling and water is running out.