The alleged attack took place on Amorella, an overnight ferry travelling from Stockholm to the Finnish port of Turku on Saturday, a popular route with a reputation for attracting Swedes and tourists who enjoy partying and can enjoy tax-free alcohol for some of the crossing.
Five were later arrested in Åbo in Finland, one was tracked down in Mariehamn and two others managed to make their way back to Stockholm before being arrested on Sunday.
By Monday afternoon, three of the suspects remained in custody in Finland and one was still being questioned in Stockholm. The other four suspects had been released, but had not been "dismissed" from the case, police spokesperson Kjell Lindgren told The Local.
"Right now the big task is gathering video evidence and witness statements," he added.
The suspects are accused of gang raping a woman, 45, who says she was approached in her cabin by a group of men.
According to witnesses speaking to Swedish newspaper Aftonbladet, the crossing was "messy", with many passengers getting drunk and a fight breaking out at a disco on board the ferry earlier in the night, with staff called to restore order.
One traveller told the paper that they saw the victim half dressed and sitting in a corridor "in a shocked state".
Other witnesses were critical of the way guards took care of the woman, suggesting that they didn't respond quickly enough or appear to show enough care or concern for her.
The Expressen newspaper claims that several of the men arrested were already known to police, and at least one had previously been convicted of handling drugs and stolen goods.
Tony Öhman, Head of Marine Operations for the ferry company, which has its headquarters in Stockholm, told The Local:
"We at Viking Line find the news about the alleged rape very tragic and sad and we are aware that issues concerning sexual assult are a bit of a problem on our cruises, however we work unconditionally with the help from the police force to prevent these crimes from happening."
He added: "We want our passengers to feel safe when they travel with our boats, we are therefore looking very seriously at what has happened. Over 6.5 million people travel on our boats safely, so I would say that the risk of something so terrible happening again is very small."
The company is one of a number of cruise operators in Scandinavia that has hit the headlines in recent years, following other alleged rapes or sexual assaults.
In 2010, 38 ferry passengers travelling in the region filed rape complaints, double the number in 2006.
Since then, police are understood to have stepped up efforts to share information about attacks among the major ferry companies, to help better identify crime trends.
A spokesperson for the Ministry for Foreign Affairs in Sweden told The Local that it was aware of the latest gang rape allegations, but that the claims were a matter for Finnish police.
Whereas Swedes are often offered consulate assistance if they are arrested abroad, a Nordic cooperation agreement means that this is not the case if they are accused of crimes in Norway, Denmark or Finland.