"It concerns a major case of suspected copyright infringement, that is to say illegal file sharing," said Paul Pintér, at the national unit for copyright crime within the Swedish police, to news agency TT.
Four Swedes have been detained on suspicion of illegal file sharing, confirmed prosecutor Fredrik Ingblad.
Ingblad did not rule out that there could be further suspects brought into the investigation.
The raid targeted homes in Eskilstuna in eastern Sweden, and in Eslöv and Malmö in the south. Police also hit two internet service providers in Solna near Stockholm, and at Umeå University in the north of Sweden.
"The purpose of the raids in the Stockholm area and in Umeå were to gather information about specific IP addresses," said Fredrik Ingblad.
He added that police also seized computers and servers in the raids.
"Certain finds have been made which strengthen suspicions."
One of the locations in Solna has been confirmed to be the premises of PRQ, which is home to WikiLeaks servers, but according to Paul Pintér the action had nothing to do with the whistleblower website.
The raids were part of a coordinated action in 14 European countries and were carried out on request of the Belgian police, according to a statement from the Swedish public prosecution office.
The investigation has been ongoing for over two years and concerns suspicions regarding the illegal file sharing of films within a network which goes by the name of "The Scene".
"The Scene" is an international network constructed in several layers.
At the top of the group are those who compete to obtain copyrighted material and publish it on the internet. The group has a leader and everyone knows each other through internet aliases. The one who shares the film is known as the supplier, who receives remuneration in cash or in "credz" - points in an internal hit list.
In the next layer of the network is found the "topsites". These are data severs, often at IT firms, where the material is held and spread through other networks. The Swedish part of the investigation is focused on these "topsites".
"It is difficult to say how many films this concerns, it is possibly in the thousands. The seizures in Sweden were done to find the servers," said Fredrik Ingblad.
He said that the crime which the Swedes are suspected of will probably be investigated by Belgian authorities.
"The seized material will probably be transferred to Belgium for investigation there. We will then have to see what comes out of that, crimes could also have been committed in Sweden."