Authorities seized about 100kg of illicit drugs and €240,000 ($252,000) in cash, after several months of intelligence gathering.
Investigators said the group had used lorries to transport more than 300kg of cocaine, amphetamines, ecstasy pills and cannabis into Finland.
“Helsinki police and the National Bureau of Investigation have uncovered a large international drug trafficking organisation,” the police said on Wednesday.
Thirty people were arrested with 17 still in custody, the force said.
The suspects are of Swedish, British, Somali, Serbian, Norwegian, Iraqi, Ukrainian, Kosovar, Turkish and Finnish nationality.
Swedish criminals were collaborating with Finnish street gangs, authorities said.
“It is a new phenomenon that Swedish organised crime is so heavily involved in the Finnish drug trade,” police in Finland said.
Sweden has in recent years been gripped by violence between rival gangs vying for control of the arms and drug trade, with frequent bombings and shootings linked to vendettas and turf wars.
Cracking down on street gangs has also been a major campaign issue for the parties that make up Finland's right-wing coalition government, which came into office in June.
“This government has recognised the rise in youth and gang crime as a serious problem,” Interior Minister Mari Rantanen said in August.
The government planned to introduce comprehensive measures by the end of the year to curb gang crime, she said.
While Finland consistently ranks as the happiest country in the world, it also has one of the highest rates of young people dying from drug-related deaths in the EU.
Between 2006 and 2021, the number of fatalities linked to illicit substances more than doubled to 287, with opioids mostly commonly to blame.
Swedish Prime Minister Ulf Kristersson vowed last week to use the military if necessary to defeat criminal gangs, who are causing innocent victims including children to suffer as a result of violence.
Reports in the Finnish media this week suggested authorities believe that Stockholm-based gang Dodspatrullen (roughly, Death Patrol) has started operating in Finland over the past year.
Detective Chief Inspector Kimmo Sainio of Helsinki police said a direct connection to Swedish organised crime is a matter of serious concern for Finnish authorities.
“Violence and firearms are a key element of drug crimes and the threshold for their use is currently very low in Sweden,” he said.
“A successful preliminary investigation plays an important role when trying to prevent the spread of international organised crime in Finland.”