Yuri Fedotov, head of the UN Office on Drugs and Crime, said at a special General Assembly meeting on trafficking that 80 per cent of trafficking victims are being exploited as sexual slaves.
"At any one time, 2.4 million people suffer the misery of this humiliating and degrading crime," Fedotov said.
He said fighting these criminals "is a challenge of extraordinary proportions".
According to Fedotov's Drugs and Crime office based in Vienna, only one out of 100 victims of trafficking is ever rescued, and two out of every three victims are women.
Michelle Bachelet, head of UN women, the new UN agency promoting women's rights and gender equality, said: "It's difficult to think of a crime more hideous and shocking than human trafficking. Yet, it is one of the fastest growing and lucrative crimes."
Criminals running human trafficking networks profit hugely from their enterprises, earning more than $32 billion (£20 billion), Fedotov told the Assembly.
Actress Mira Sorvino, the UN goodwill ambassador against human trafficking, said: "Modern-day slavery is bested only by the illegal drug trade for profitability."
She said: "Transnational organised crime groups are adding humans to their product lists. Satellites reveal the same routes moving them as arms and drugs."
But, the actress pointed out, compared with efforts to subvert drug traffickers, little money and political will is being spent to combat human trafficking.
Speakers at the daylong meeting also highlighted the need to make human trafficking a priority of crime fighting agencies.
M. Cherif Bassiouni, an emeritus law professor at DePaul University in Chicago, told the assembly: "There is no human rights subject on which governments have said so much but done so little."
The international UN war crimes expert said that current laws target the prostitutes and other victims of crime rather than the perpetrators "without whom that crime could not be performed".
Bassiouni added that the figure – 2.4 million people trafficked at any time – underplays the scale of human trafficking across the globe, predicting that the actual number of victims at the end of a 10 year period will be significantly higher.
"We must change attitudes of male-dominated police departments throughout the world who place this type of a crime at the lowest level of their law enforcement priorities," he said.
General Assembly President Nassir Abdulaziz Al-Nasser and Secretary-General Ban Ki-moon encouraged UN member states to donate to a new trust fund aimed at helping victims of human trafficking.