Officers arrested 19 people - three of whom are the alleged linchpins of the network, and others key lieutenants - when they stormed 12 houses in north, east and west London and one in Boston, Lincolnshire. Scotland Yard chiefs believe the ring is the biggest criminal enterprise of its type they have encountered. Police sources described the multimillion-pound racket, which involved hundreds of handlers smuggling tens of thousands of people - the vast majority Turkish Kurds - across Europe into Britain, as "absolutely massive" and "frightening" in scale.

Yesterday morning's arrests were the culmination of a two-year investigation, codenamed Operation Bluesky, by the Metropolitan police's Maxim unit, which tackles organised immigration crime. Detective Chief Superintendent Bill Skelly, one of the lead officers, said: "This is the biggest operation of its kind that the Met has been involved in."

More than 200 officers carried out the raids. Those arrested are thought to be mostly Turkish Kurds. Some came to the UK several years ago as asylum seekers and were granted indefinite leave to stay.

In London, five premises were raided in Enfield, two in Bexleyheath, and one each in Barnet, Hackney, Tower Hamlets, Haringey and Hammersmith. A police dog was brought in to help search one of the houses in Bexleyheath. In the past three years, the gang is thought to have lured thousands of economic migrants from Turkey to the UK with the promise of a better life.

The trips often took months, with the illegal immigrants sometimes travelling hundreds of miles, as many as 20 of them lying flat, barely able to move, in cramped, secret compartments in lorries, and paying £3,000 to £5,000 each for the journey. "It's a tortuous journey, full of discomfort and danger, but they are determined to get here, given the particular attraction of London's established Turkish community," said a senior police source.

Once in the UK, the migrants usually headed straight for north-east London and disappeared into the large Turkish/Kurdish community, where many have strong family ties, getting low-paid black market jobs in local cafes and shops. Some were given stolen or forged UK identity documents, and many used their wages to bring more relatives to Britain.

The Met is working closely with police forces in France, Italy, Holland, Belgium and Denmark, and Europol. There have been 20 arrest operations in the UK and some of these countries in the past couple of years to disrupt the network, with those convicted receiving sentences of between two and 10 years in jail.

But senior officers now believe they have the key figures. Assistant Commissioner Tarique Ghaffur, head of Scotland Yard's specialist crime directorate, said yesterday's raids were aimed "right at the top of this network." Mr Ghaffur said: "Our commitment is to take out such networks and this is the latest in our collaboration with the growing number of law enforcement agencies in Europe to work robustly to achieve this mission."

UK-based relatives of those wishing to come to Britain generally approached people smugglers in London, paying some cash up front and the balance on the arrival of their family member. The smugglers then organised a complex chain, passing a group of illegal immigrants from handler to handler across Europe into Britain, often an in arduous process lasting months. They typically travelled from Turkey up through the Balkans and across to Germany and Italy in lorries, continuing to Belgium or France, sometimes by train, where they would hide out in safe houses near major ferry ports such as Calais, Cherbourg or Dieppe until the smugglers deemed the time was right to shuttle them to the UK. Some entered England by ferry, in lorries, vans or private car, and some by light aircraft into small airports in Kent and Cambridgeshire. Detectives believe many were brought to the UK up to 20 at a time in secret compartments in lorries, with each "cargo" worth around £100,000 in smugglers' fees.

Those at the top are thought to have made huge personal fortunes. Some of the money has gone into cafes, snooker halls and other businesses in London, but police are still trying to trace a huge flow of money out of the UK. Eight of the 19 arrested are suspected of facilitating human smuggling. Detectives are questioning six about immigration offences, and one about money laundering. Police also seized tens of thousands of pounds in cash. Two of those in custody are women, accused of interfering with the inquiry.