The demon invaders where very effective, they rounded people up and put them under lock and key. They used anything that they found effective, from jails, to schools, public buildings but their favorite places to set up where places of religious worship.
Once they had set up then the process began, like slave auctions of the past. Demons would sell the captive humans to other demons. They would sell the humans in any way they saw fit, in pairs, family units, small and massive lots, if a human had a special skill they wanted they sold them alone.
The demons where twisted in many ways, they used the old human network of commutations to broadcast the auctions live over the TV and then let the people in captivity watch. People watched as once bought people where restrained and then a hooded demon placed his or her hand on the victim which began to process of turning the victim into something the demon who bought them wanted.
The most common transformation was into a slave, in which they victim eyes would roll over as glowing band a collar appeared around their neck and they would be becoming a mindless slave who would follow any orders of their new owner. Whole cities became mindless slaves.
Others became demons of different classes and intelligence, some even remained human but now worked for the demons to build their new world. Others where soldiers who blindly followed orders. Physically while the turned humans looked no different then the demons from hell all of them where slaves in some way to the “pure” demon owner. Families where broken apart and the demons didn't really care about keeping families or couples together unless it helped raise the price of the sell.
Some became maids and servants to their new owners, some warriors, some even sex demons for their owners. That is what happened to the President on live TV the President was sold and then transformed into a Bimbo succubus ready to serve his well now her new master. Physical, sex, race and age changes where all common.
Now you sit in a cell waiting for your turn.