A top-secret communications system used by criminals to trade drugs and guns has been "successfully penetrated", says the National Crime Agency.
Major crime figures were among 746 arrests after messages on EncroChat were intercepted and decoded.
More than two tonnes of drugs, several dozen guns and £54m in suspect cash have been seized, says the NCA.
The NCA worked with forces across Europe on the UK's "biggest and most significant" law enforcement operation.
Officers are said to have prevented people being murdered after covertly monitoring planned attacks and threats to life on the encrypted service.
The NCA says the Europe-wide operation, which lasted over three months and involved police forces across the UK, has had the biggest impact on organised crime gangs it has ever seen.
The Metropolitan Police Commissioner, Dame Cressida Dick, whose force made 132 of the arrests and seized £13.3m in cash, described it as a "game changer".
She said: "This is just the beginning. We will be disrupting organised criminal networks as a result of these operations for weeks and months and possibly years to come."
An estimated 60,000 people, among them up to 10,000 in Britain, subscribed to France-based EncroChat, which has now been taken down.
The system operated on customised Android phones and, according to its website, provided "worry-free secure communications".
Customers had access to features such as self-destructing messages that deleted from the recipient's device after a certain length of time.
There was also panic wipe, where all the data on the device could be deleted by entering a four-digit code from the lock-screen.
The NCA says the messaging system has been used as a "criminal marketplace" to co-ordinate the supply of Class A drugs across the world, and import weapons including assault rifles, sub-machine guns, shotguns, pistols and hand grenades.
Law enforcement agencies began getting data from the site on 1 April after the encryption code is believed to have been cracked in March.
Gangs are also believed to have used the handheld devices to plot attacks on rival groups, plan ways of enforcing drug debts and arrange for money to be laundered.
Threats to life detailed on the site included acid attacks and threats to chop off limbs.
Dozens of organised crime groups have been dismantled, says the NCA, with the bulk of arrests in London and north-west England.
Lockdown "worked in our favour", says the agency, in that many more suspects were at home when they were raided.
Many of those arrested are said to form the "middle tier" of crime gangs while some are described as the "Mr and Mrs Bigs" of the underworld.
Chat users realised 'too late'
Analysis by BBC technology reporter David Molloy
EncroChat sold encrypted phones with a guarantee of anonymity, with a range of special features to remove identifying information. The phones themselves cost roughly £900 (€1,000) each, with a subscription costing £1,350 (€1,500) for six months.
Europol said that French police had discovered some of EncroChat's servers were located in the country, and that it was possible to put a "technical device" in place to access the messages.