Known as the almajiri, the youngsters, some no older than 5, have flooded the streets for the nearly 15 years since a tsunami of cheap Chinese imports and a dysfunctional electrical system began destroying the region's once-thriving textile manufacturing industry. Many more children have streamed in from rural areas since similar collapses of the fishing and agriculture sectors left their parents unable to feed them.
Here, the youngsters' only schooling comes from threadbare clerics for whom they recite the Koran a few hours a day. And then they go out and beg for food. Such conditions have made the almajiri prime fodder for recruitment by Boko Haram.
"These are ... vulnerable children," said human rights activist Shehu Sani, an expert on the violent Islamist group that emerged in 2002. "They have in many cases turned to extremism and crime because they were sent away by their parents at a very tender age and they grow up under the care of teachers who use them."
Among the almajiri, said Sani, was Abubakar Shekau, current leader of Boko Haram, which initially mushroomed by offering a religious path that condemned what it called the corruption and decadence of Nigeria's Western-educated elite. After its founder, Mohammed Yusuf, was killed in 2009, the group, led by Shekau, became increasingly violent, slaughtering non-Muslim schoolboys, kidnapping hundreds of rural schoolgirls and shooting down villagers by the dozen with AK-47 assault rifles.