North and South Yemen were separate countries until 1990 and fought a brief civil war four years later. For decades, southern grievances over ill treatment by the rulers in the northern capital, Sana, have festered, escalating in recent years into a movement openly calling for secession.

Professor Shuaibi was among thousands of people protesting peacefully a few months ago in Aden for an independent state, in a square adorned with pictures of southerners who had died in wars and at the hands of Mr. Saleh’s security forces.

On Thursday, he was back in the streets with a gun, preparing to join other residents fighting in the central district of Al Mualla.

The local militias are loosely organized, dominated by young men focused on securing their own neighborhoods, said Jamal Khulaqi, a 25-year-old Yemeni-American from Buffalo who said he was helping with relief efforts in the city. Most lack training and weapons apart from AK-47s.

Their opponents are mostly security men loyal to Mr. Saleh, known for their repression back when they were in power. Now, as militiamen, residents say, they are unrestrained and more brutal. “They are bombing innocent people, families,” Professor Shuaibi said.

The Houthis, fighting all over the country, are a smaller part of the force in Aden, their ranks filled with many teenagers and even some children. Some of the young Houthis who had been captured seemed filled with religious zeal and said they had been told they were going to Aden to fight Al Qaeda, the Sunni extremists the Houthis regard as their principal foe, Professor Shuaibi said.

“There is no Al Qaeda here,” he said.

The city has been carved up into sectors guarded by fighters with guns slung over their shoulders, drained by the stresses of war but still full of swagger. One fighter, Mohamed Saleh Salem, 38, called the local fighters “ferocious” and vowed that the Houthis would not advance, while adding that he had not had a bath in days.