Its Office on Drugs and Crime estimated more than 140,000 people are controlled by organised gangs. Many victims are tricked into leaving lives of poverty in eastern Europe, Africa and Latin America with bogus promises of work.
"Europeans believe slavery was abolished centuries ago. But look around - slaves are in our midst," the a Antonio Maria Costa said. Costa said one big problem is that governments in industrialised countries have only recently passed tougher laws to crack down on trafficking in people.
"It is a very recent recognition of a very old problem," He added that arrests and convictions of traffickers are rare. "I could count them on one hand."
Worldwide, his agency estimated several million people have fallen victim to traffickers.
American actress Mira Sorvino, who serves as a goodwill ambassador for the UN agency, said she met women in Madrid who have been rescued from trafficking gangs in Spain and their stories were heartbreaking.
One Romanian woman was beaten so badly while being smuggled to Spain that her ribs were broken. Despite the injury, she still had to service clients in a roadside brothel while she recovered, Sorvino said.
Another woman, from Nigeria, was fooled into travelling to Spain with a promise of work so she could support her daughter back home.
After traveling to Spain in the cargo hold of a ship, and seeing several travel mates die along the way, the woman learned there was no work waiting for her. She ended up as a prostitute and was told she had a ?euro50,000 debt to pay off.
People back in Nigeria who had promised to care for her daughter instead had a chilling new message.
"If you do not pay, we will kill your daughter," Sorvino quoted the woman as recalling.