Eight people have been arrested as part of an investigation into a gang which has been staging bogus road accidents.
The nine-month investigation culminated in dawn raids at addresses in Watford, Hatfield, Isleworth and Birmingham.
Those arrested are being held on suspicion of conspiracy to defraud insurance firms by staging fraudulent accidents - known as "cash for crash".
The scam involves offenders causing crashes between themselves and innocent motorists and making a false claim.
The investigation, codenamed Operation Mysterious, involved a specialist team of detectives within Hertfordshire Police. It began following information received about a major insurance scam operating in the south of the county.
Substantial sums of money are thought to have been made by those responsible for the scam.
John Beadle, chairman of the Insurance Fraud Bureau, said: "The success of this operation sends a very clear message to those criminals who are targeting innocent motorists, we know who you are and we are coming after you."
The eight arrested are being questioned at police stations across Hertfordshire.