Twenty people have been arrested in a series of dawn raids to crack down on a major student visa immigration scam.
The raids were carried out by the Operation Maxim team
More than 120 officers from Scotland Yard's specialist crime directorate searched 12 addresses across London and Essex.
These included two suspected bogus colleges in Tooting in south London.
The raids aim to end an immigration racket estimated to have brought about 1,000 people into London and earned the perpetrators millions of pounds.
Wednesday morning's operation, codenamed Taming, was the largest yet to be carried out by the Metropolitan Police under the banner of Operation Maxim, an initiative to tackle organised immigration crime in London.
The alleged fraud involves producing fake documents claiming immigrants are studying at the Tooting colleges so that they can obtain student visas giving them leave to remain in the UK.
Most of the immigrants targeted are believed to be South Africans.
The raids were also timed to coincide with activity by the South African police service in Durban, where officers were thought to have visited at least one address.
London officers also targeted addresses in areas including Upper Norwood, Anerley, Mitcham in south London, Canning Town, north London and Westcliff-on-Sea, Essex, during the raids, which were supported by the Immigration Service and UK's Passport Agency.
Assistant Commissioner Tarique Ghaffur, who is responsible for Maxim, said: "This is the largest investigation conducted so far by Maxim and highlights the Metropolitan Police's determination to counter organised immigration crime in London.
"Today I want to make it absolutely clear that we will take a robust stance against those abusing UK immigration laws."
Those arrested have been detained on suspicion of offences relating to facilitating illegal entry and leave to remain in the UK, and money-laundering and are being questioned at various police stations in London.
A financial investigation is also under way to retrieve the assets of those involved.