The operation was launched in the early morning in Stockholm and in southern Sweden, Stockholm city police spokesman Kjell Lindgren told AFP.
Swedish police began investigating the group after noticing a number of new Romanian prostitutes in August on Malmskillnadsgatan, a Stockholm street known as a hangout for hookers.
“The girls who are working as prostitutes come from Romania, as do the people who were arrested,” Lindgren said, adding that the nine arrested were eight men and one woman.
“The crime they are suspected of is aggravated pimping, but we think that it may be a question of human trafficking given that the girls are under the age of 18,” he said.
The young women and the nine people arrested were to be questioned by police before a prosecutor decides whether or not to remand the nine in custody.
Police were also to question seven other people, who were not arrested, in the case.
Lindgren said it was too early to speak of a prostitution ring, but the nine people arrested were working together “in an organised manner”.
The spokesman said the operation was continuing on Tuesday and further arrests were possible.