Images of rebellion have sprung up across this southern coastal city. The old flag of South Yemen is proudly displayed on cars, and graffiti calling for independence is sprayed on wall after wall.

“Freedom for South. Aden Get Up,” reads one message along a busy main road.

Once mostly underground, a secessionist movement seeking to undo a 1990 pact that unified North and South Yemen is emerging from the shadows, emboldened by the populist uprising that has upended the country over the past year.

Many of its members believe that they helped inspire the uprising when they took to the streets in 2007. “We are the ones who started the revolution,” said Said Quhail, a leader in the movement.

Yemeni security and intelligence services long targeted those who supported secession, using beatings, arrests and torture, and driving many to flee into exile or live clandestine lives. But with the central government weakened by political turmoil, the southern secessionists have sensed an opportunity to overturn what they describe as years of marginalization — and to create a separate South Yemen again.

Today, they hold public rallies and conferences in five-star hotels, and fly in and out of the country.

“We are becoming stronger and stronger because of the political situation,” declared Nasser Al Tawil, a retired general in South Yemen’s army and a top secessionist leader. “Our point of view is becoming stronger and stronger.”

In recent weeks, top diplomats from the United States, Britain and the United Nations have urged the movement’s leaders to join Yemen’s political process. Among other concerns, they worry that al-Qaeda’s ambitious Yemen branch, which is based in the south and has targeted the West, could take advantage of the turmoil. It was here in the port of Aden that al-Qaeda attacked the USS Cole in 2000, killing 17 American sailors.

President Ali Abdullah Saleh left the country Sunday and headed to the United States for medical treatment, potentially clearing the way for a transfer of power after more than 30 years. But what his departure means for the secessionist movement is unclear. His top advisers insist that Saleh will return to Yemen to lead the ruling party, possibly ensuring that he will continue to wield influence in the months and years ahead.

Tensions have existed between northerners and southerners since Yemen unified, and a brief civil war broke out in 1994, ending in victory for the north.

Southerners say Saleh and his northern tribesmen have denied them their share of oil revenue; about 80 percent of Yemen’s oil production is located in the south. They say the administration dismissed many southerners from military and government jobs, denying them access to even local power, and point out that the governors of all seven southern provinces are from the north. Southerners also accuse influential northerners of grabbing land in the south for personal gain.

The movement’s more radical leaders want an immediate separation from the north, returning to the pre-1990 geography. Moderate leaders seek a federal system, in which more power would be devolved to the south. After five years, a referendum on self-determination would decide whether the south would remain part of a united Yemen or secede, much like what took place in South Sudan last year.

With Saleh agreeing to cede power, Arab and Western diplomats worry that a failure to address the south’s grievances could handcuff Yemen’s transition, the worst-case scenario being another civil war.

Jamal Benomar, the U.N. secretary general’s special adviser on Yemen, said in a recent interview with the Yemen Times newspaper that he was concerned about the southern movement’s “fragmented” political views. He said its “lack of inclusiveness” in the political process could be harmful to the country’s transition.

Tawil, the secessionist leader, said little will change until Saleh and his family give up power completely. But the movement feels that it no longer has to operate secretly, he said, and that it can never be boxed in again.

In 2009, he met with this reporter inside a white car pocked with dents. He and another secessionist leader looked nervously out the window, watching for Yemeni intelligence agents. They insisted that they drive around the city during the interview.

Last month, Tawil spoke outside his apartment about his desire for self-determination. When a Yemeni soldier walked past him, he didn’t run away or even flinch. He was planning a large public rally in a few days, he said.

“We will not stop,” Tawil vowed. “We are going to escalate our political activism in a greater way. We want our rights.”