The operation known as Trojan Shield led to police raids in 18 nations. More than 800 suspects were arrested and more than 32 tonnes of drugs - including cocaine, cannabis, amphetamines and methamphetamines - were seized along with 250 firearms, 55 luxury cars and more than $US148 million ($A191 million) in cash and cryptocurrencies.

The seeds of the sting were sown when law enforcement agencies earlier took down two encrypted platforms, EncroChat and Sky ECC, that had been used by criminal gangs to organise drug trafficking and underworld hits.

With the gangs in the market for a new means of communication, the FBI stepped in with a covertly developed app called ANoM that was installed on modified mobile phones.

In the past 18 months, the FBI provided phones via unsuspecting middlemen to more than 300 gangs operating in more than 100 countries.

Intelligence gathered and analysed "enabled us to prevent murders. It led to the seizure of drugs that led to the seizure of weapons. And it helped prevent a number of crimes," Calvin Shivers, assistant director of the FBI's Criminal Investigative Division, told a news conference on Tuesday in The Hague, Netherlands.

The operation - led by the FBI with the involvement of the US Drug Enforcement Administration, the European Union police agency Europol and law enforcement agencies in several countries - dealt "an unprecedented blow to criminal networks, and this is worldwide", said Dutch National Police Chief Constable Jannine van den Berg.

Australian Federal Police Commander Jennifer Hearst called it "a watershed moment in global law enforcement history".

The ANoM app became popular in criminal circles as users told one another it was a safe platform. All the time, police were looking over the shoulders of criminals as they discussed hits, drug shipments and other crimes.

"There was a void that was created by a lack of these encrypted platforms," Shivers said, of the initial move to take down apps previously used by gangs.

"So that created an opportunity for collaboration with our international partners, to not only develop the specific tool but also to develop the process of gathering the intelligence and disseminating the intelligence."

Law enforcement agencies from Sweden to New Zealand described the operation as having a significant impact.

Swedish police prevented a dozen planned killings and believe they have arrested several "leading actors in criminal networks", according to Linda Staaf, the head of Sweden's national criminal intelligence unit.

Finnish police said nearly 100 people have been detained and more than 500 kilograms of drugs confiscated, along with dozens of guns and cash worth hundreds of thousands of euros.

In Germany, the general prosecutor's office in Frankfurt said more than 70 people were arrested on Monday and drugs, cash and weapons were also seized.

In Australia, authorities said they arrested 224 people and seized more than four tonnes of drugs and $US35 million.

New Zealand police said they had arrested 35 people and seized drugs and assets worth millions of dollars.

European police last year delivered a major blow to organised crime after cracking an encrypted communications network, known as EncroChat, used by criminal gangs across the continent.

In March, Belgian police arrested dozens of people after cracking another encrypted chat system and seizing more than 17 tonnes of cocaine.

The latest operation went even further.

"The success of Operation Trojan Shield is a result of tremendous innovation, dedication and unprecedented international collaboration," Shivers said. "And the results are staggering."

Australian Associated Press