The number of reports involving the exploitation of mentally handicapped women has also increased, according to a report on human trafficking from Sweden's National Police Board (Rikspolisstyrelsen) released on Tuesday.
"Alarming," Kajsa Wahlberg of the police board told news agency TT.
Last year was the first time Nigerian women were reported to have been involved in the sex trade in Sweden, likely as a result of the introduction of a law in Norway prohibiting the buying of sexual services that came into effect on January 1st, 2009.
In addition, the number of exploited Thai women has increased. There were also several reports of the pimping of exploited mentally handicapped girls last year.
The trafficking of Nigerian prostitutes has previously been a large problem in the rest of Europe. In Sweden, however, they have only appeared on a small scale in Gothenburg.
A trial is currently under way in Stockholm exposing a Nigerian prostitution ring. The exploited women involved were forced to undergo a voodoo ritual in their home countries, where they were intimidated into submission and repaying debts to the masterminds behind the ring.
The police board estimates that about 50 Nigerian women were exploited in the sex trade in Sweden last year. They often lack education and many of them are illiterate.
The Thai women who come to Sweden and are exploited for sexual purposes often come here through marriage or on visitor visas after being invited by Swedish men who then sell them into work in apartments, hotel rooms or at Thai massage parlours.
In the Stockholm area alone, police estimate that there are about 90 Thai massage parlours.
"Most offer sexual massage," said Wahlberg.
The police have never seen so many cases of sexual exploitation of girls and women with mental disabilities as were reported last year. Cases have been reported by police authorities in Stockholm, central Gävleborg and southern Skåne.
"They are totally unscrupulous men who exploit these helpless girls," said Wahlberg.
Other facts from the report reveal that the majority of women who were trafficked to Sweden in 2009 came from eastern Europe, Russia, Estonia, Romania, Hungary and Albania.
Most of the traffickers were men, but some were women, coming from Romania, Moldova, Ukraine, Sweden, Estonia, Nigeria and Thailand.
There were also examples in which the traffickers exploited the victims for several purposes. In one case, two Czech women were trafficked to Sweden to steal during the day and prostitute in the evenings and nights.