ADEN, Yemen — For all the challenges that Yemen’s new president faces, none may be more imperative than the unsettled state of the south, where many are eager for secession and a security breach has allowed an Al Qaeda affiliate to grow strong.

The president, Abdu Rabbu Mansour Hadi, has moved quickly to try to shore up the south amid rising violence and political uncertainty. He appointed a new head of security and a new governor for the southern province of Aden, as well as a new commander of the southern military force. But residents of the south say that while shifting personnel may help in the long term, the crisis needs to be addressed more aggressively now.

“It couldn’t be worse,” said Sheikh Tariq Abdullah, who has practiced law in Aden since British colonialists were here over 50 years ago. “A very, very large majority of southern Yemenis of almost every class, they just don’t want unity. What the street is saying is that they are absolutely fed up.”

Yemen is beginning to assess and deal with damage to the economy and social fabric after a nearly yearlong public uprising against Ali Abdullah Saleh, the president, who governed for three decades before being ousted. Mr. Saleh’s agreement to relinquish power was regarded as a prerequisite, not an answer, to resolving challenges facing a nation that even before the crisis embodied characteristics of a failed state.

That is especially true in the south, where the political transition has not only done little to stabilize the situation but has also magnified longstanding complaints that southerners have been marginalized politically, economically and socially by the northern government since the unification of North and South Yemen in 1990. That sentiment, along with the growing strength of the Qaeda affiliate Ansar al-Sharia, has created a volatile environment.

“You cannot solve the problem partially,” said Mohammed Bashraheel, a well-known resident who says the political and security issues must be resolved simultaneously. “It is a package.”

In this port city, once the capital of an independent nation, crime is rampant, the rule of law has largely been abandoned and graffiti that declares “Free the occupied south” is scrawled on the walls of colonial-style buildings. For example, residents complain that they cannot go to the police to report crime because gangs have burned the police station down.

“When the government fails, then what happens? The people rise up,” said Bar al-Maqtha, a southern separatist advocate who was in the Mansoura neighborhood, an area of Aden where the police do not venture because the separatists are in control. “We refuse the elections. We refuse Abdu Rabbu. We want to the end occupation of south Yemen.”

Though many southern separatists remain committed to dissent by peaceful means, even in the face of police brutality, the past year has been marked by an increase in violent revolt. Afternoon shootouts between security forces and separatist gangs, which mainly consist of unemployed young men, are common in some Aden neighborhoods, like Mansoura or Sheikh. In one such episode, gunfire rang out as residents scurried into white concrete apartment complexes from the Soviet era.

“You see, when I get on the bus in the morning, maybe the people will say at the station, ‘Don’t go to Sheikh, don’t pass that way, because there’s a gun battle there,’ ” said Omar Mohammed, a 26-year-old Aden resident. “It’s one step forward and two steps back in the fighting.”

Other separatists complain that the armed gangs consist of unruly Bedouins from the surrounding countryside, a reminder that the south itself is far from a monolithic unit and that the secessionist movement is exceedingly fractured.

Fueling a widespread sense that the government has turned its back on the south is the more immediate concern over security. In many areas, the security forces pulled out a year ago. That, residents say, has led to a dire situation in which Ansar al-Sharia, the affiliate of Al Qaeda, has appeared increasingly brazen — and violent. In Al Qaeda’s deadliest attack ever on Yemen’s military, Ansar al-Sharia took credit for killing more than 100 soldiers recently at a military post just west of the capital of Abyan Province, Zinjibar, about 30 miles northeast of Aden. The militants say they have captured other soldiers.

Perhaps more troubling, Ansar al-Sharia is functioning as a de facto government in small cities and towns in Abyan and Shabwa Provinces, and it is winning the loyalty of at least some of the population, according to residents.

The militants are working from an established playbook, offering the services that the government has failed to provide. They are giving food rations to civilians and providing some sense of security with their own police force and court system based on strict interpretations of Islamic law.

“When someone takes over and says I want to rule by the Shariah, no one can stand against them,” said Ahmed al-Fadhli, who comes from a powerful family in the area of Abyan Province controlled by the militants. “That’s real justice.”

It is a shift in targets for the terrorist group, which has thus far set its sights on Yemeni military units, rather than staging attacks that hurt the local population, like the marketplace bombings conducted in other Arab countries.

“We want security, whether it is from Al Qaeda or the government,” said Ahmed Saeed, a father of eight who recently came to Aden with his family after the militants told them there might be an attack soon on Jaar, in Abyan Province. Like thousands of others displaced by fighting in the province, Mr. Saeed and his family set up residence in decrepit public school buildings.

While Yemenis in the north are hopeful that Mr. Saleh’s recent resignation offers a fresh start for their struggling nation, most southerners say they do not want anything to do with the new political agenda.

And though there are many posters of Mr. Hadi hung on walls across Sana, the nation’s capital, in Aden there are flags of the former South Yemen painted on buildings, pasted on telephone polls and strung over streets.

“The problem is that when the young guys, when they were growing up, they were told how their family were getting jobs. Now they see how their parents have been humiliated,” said Yahya al-Jifri, a leader in an opposition political party in Aden. “They find out they cannot go to university because there are no scholarships for southerners. Then their fathers lost their jobs in military and civil service.”

Back in his law office in central Aden, Mr. Abdullah gave a pessimistic assessment of where he sees all this leading. “There is no central leadership,” he said. “We have no economy. There is no foundation anymore for an independent country. We will just end up fighting among ourselves.”