The 10 men and 11 women were held during raids on nine addresses across London shortly after dawn.
Police also seized several items which might be linked to ritual killing, including an animal’s skull with a nail driven through it.
Earlier this month they arrested a 37-year-old Nigerian man in Dublin in connection with child trafficking and with Adam's murder. That arrest led to Tuesday's raids.
Scotland Yard police commander Andy Baker said he believed there was "a very strong link" between the suspects and the murder of the young boy – named Adam by police – whose mutilated body was found floating under London's Tower Bridge in September 2001.
The boy's head, arms and legs had been hacked off and his torso was clad only in a pair of orange shorts. Experts said he was almost certainly the victim of a ritual killing.
Police still have no idea who he was, except that he was probably four or five years old.
"I am pretty confident that we're on to a group of individuals who would have trafficked Adam into the country," Baker told a news conference.
The search for the killers took officers to Germany, the Netherlands, the United States, South Africa, West Africa, Scotland and Ireland, but yielded few clues.
At one point they believed he was the victim of a South African "muti" ritual slaying. Former South African President Nelson Mandela made an appeal across Africa for anyone with information about Adam to come forward.
Traces of bone and minerals, found in the boy's stomach, led police to Nigeria and from there to Ireland.
Baker said police did not yet know the names, ages and nationalities of all those arrested and were still carrying out tests on items and substances seized