Has the end finally come for President Ali Abdullah Saleh after nearly 33 years in power? Will he ever return from Saudi Arabia where he was rushed for treatment for his injuries a day after an explosion in a mosque in the presidential palace?

The crowds celebrating the end of President Saleh's rule in the capital, Sanaa, yesterday are probably right that he is finished, though he has shown extraordinary determination over the last four months to cling to office.

Pundits ask if the vacuum of power following his departure will see the country plunge into civil war. This is possible, but there has always been a vacuum of power in Yemen, where the state is historically weak. Despite the escalating violence between the President's supporters and the opposition – both peaceful protesters and armed tribesmen – a deal remains feasible.

Much depends on how badly President Saleh is injured and how swiftly his support base unravels inside Yemen and the attitude of Saudi Arabia and the US outside it.

Elite military units are under the command of his son, nephews and half-brothers, though other parts of the army have defected to the side of the protesters in the months since they held their first rally on 27 January, and particularly since pro-regime gunmen shot dead 52 demonstrators in Sanaa on 18 March. Much of President Saleh's support, built up through his extended family, allied tribes and control of official jobs over three decades, depends on him personally and without his presence may swiftly disintegrate. There were unconfirmed reports yesterday of top ministers and other officials trying to flee the country with their families.

The attitude of Saudi Arabia, for long President Saleh's chief supporter and paymaster, will be crucial. It is difficult to see how he can return to Yemen without their assent and co-operation, and why they should give it.

The US, also a long-term ally of the old regime, is looking for a smooth transition. The Vice-President, Abed Rabbo Mansour Hadi, who constitutionally takes over from the head of state when he is out of the country, saw the US ambassador in Sanaa yesterday.

The transition to a post-Saleh government will be more fraught than it would have been if it had taken place when the protests began. Since then violence has increased and there are many soldiers and gunmen on opposing sides who might decide to fight it out. The peaceful protesters have also been pushed to one side by opposition tribal leaders, led by Sheikh Sadeq al-Ahmar, the leader of the Hashid tribal federation, and the military leaders who defected to the opposition.