NAIROBI, Kenya — Al-Shabab Islamist rebels abruptly pulled out of Mogadishu, the bullet-ridden capital of Somalia, on Saturday, leaving the entire city in the hands of the government for the first time in years and raising hopes that aid groups could now deliver aid to more famine victims unfettered.

Witnesses described truckloads of heavily armed al-Shabab fighters driving away under the cover of darkness and beleaguered residents pouring into the streets to cheer and jeer their departure.

"We have been dreaming of this day for the last three years," said Abdiweli Mohamed Ali, Somalia's new, Harvard-educated prime minister.

Al-Shabab's notoriously brutal brand of Islamism has tormented Mogadishu residents for years, but more recently the rebels have also blocked international relief groups from bringing food to victims of the famine that has swept southern Somalia this summer, contributing to tens of thousands of deaths.

Aid groups hoped al-Shabab's retreat would allow them access to more parts of the capital, where more than 100,000 famine victims have come seeking aid. But al-Shabab still controls large parts of southern Somalia, the areas worst hit by drought and famine.

The rebels said Saturday that they had "completely vacated Mogadishu for tactical purposes," according to Ali Mohamud Rage, an al-Shabab spokesman. He said that al-Shabab would change its strategy to "hit-and-run attacks."

"We will be back soon," he warned.

But in the past few months, al-Shabab, who have pledged allegiance to al-Qaida, have taken a beating in steady urban fighting against a better-armed, 9,000-strong African Union peacekeeping force. Many analysts have said al-Shabab was growing weaker by the day.

Al-Shabab has imposed a brutal reign in areas it controlled, chopping off the hands of petty thieves, lashing women for showing their ankles and beheading anyone deemed a spy. They also banned music, television, gold teeth and even bras, branding them all un-Islamic.

The rebels have been divided over whether to let in Western aid organizations to relieve the famine. There are indications, too, that they are running out of cash.

Al-Shabab has prevented most Western aid groups from bringing life-saving help to the rural areas of southern Somalia it still controls. Al-Shabab are also blocking starving people from leaving their territory; al-Shabab fighters have set up their own large displaced persons camp about 25 miles from Mogadishu where they are essentially imprisoning families trying to escape al-Shabab territory.

Al-Shabab's departure from the capital offers no guarantee that Somalia's weak transitional government will be able to capitalize on the opportunity, or that the city's population will rally behind the government. Somalia's Transitional Federal Government (TFG) has been propped up by millions of dollars of Western aid, including U.S. military aid, but its leaders remain ineffectual, divided and by many accounts corrupt.

"Unfortunately, I entertain no delusions that the TFG will rise to the occasion," said J. Peter Pham, Africa director at the Atlantic Council, a Washington-based policy institute. "The only exploiting that will be taking place will be whatever schemes its ministers will hatch to profit from increased aid flows."

As the nation faces one of the worst droughts in 60 years, many analysts are pessimistic that the government will be able to deliver. On Friday, government troops looted sacks of grain and killed several people during a riot over emergency food in a refugee camp.

Several Mogadishu residents interviewed on Saturday were happy al-Shabab was gone but a bit tentative about what it meant.

"It was good they left because they were very oppressive," said Mohamed Yare, who was arrested by al-Shabab a week ago for talking about soccer. "But, the government must come with policies to restore the security of the area abandoned by the Shabab, and if the government forces start looting cellphones and other properties," which has happened many times before, "the Shabab might get a vacuum to return."

Maj. Paddy Ankunda, a spokesman for the African Union troops in Somalia, said the peacekeepers were cautious "because it could be a trap."

Witnesses said al-Shabab fighters were heading south of Mogadishu toward Merca, Brava and other towns they control in southern Somalia. In late 2006, the militant wing of the Islamist movement that had ruled Mogadishu made a similar escape, driving all the way down to the Kenyan border where it melted away into the bush, only to regroup a few months later.

Residents of Mogadishu also said that emissaries of various warlords were already beginning to identify bases in the neighborhoods that al-Shabab had just vacated, which could spell yet another problem for the government.