After a hail of machine-gun fire in the late afternoon in downtown Tunis, snipers were visible on the rooftop of the Interior Ministry, aiming down at the Boulevard Bourguiba. Human rights groups said that they had confirmed dozens of deaths at the hands of security forces even before the biggest street battle began Friday, and on Saturday residents huddled in their homes for fear of the police.

The tumbling political succession started Friday when Prime Minister Mohamed Ghannouchi announced on state television that the president was gone and that he was taking over. Then, on Saturday morning, Mr. Ghannouchi, an ally of Mr. Ben Ali, abruptly announced that he was surrendering the reins of government to the speaker of Parliament, complying with succession rules spelled out in the Tunisian Constitution. Now the speaker, Fouad Mebazaa, is expected to hold elections to form a new government within 60 days…

But others at home and abroad worried that Tunisia could slide into chaos, laying the groundwork for a new strongman to emerge. Mr. Ben Ali was viewed in the West as a reliable ally in the fight against the Islamic extremism flourishing in other parts of North Africa, and in Washington, national security experts said extremist groups like Al Qaeda in the Islamic Maghreb could capitalize on the disorder to find a new foothold.

For now, though, the political field remains conspicuously empty. Mr. Ben Ali’s pervasive network of secret police had succeeded in effectively eliminating or co-opting any truly viable opposition or political institution. The former president also long ago wiped away the Islamist groups that form the main grass-roots opposition in most Arab countries.