A total of 143 FSA employees took part in the operation
Six people have been arrested on suspicion of taking part in a long-running insider dealing scheme.
The arrests came after 16 addresses were searched in a joint Financial Services Authority (FSA) and Serious Organised Crime Agency (Soca) probe.
Documents and computers were seized from premises in London, the South East and Oxfordshire in the FSA's biggest operation against insider dealing.
The joint operation, the first between the two agencies, began in 2007.
Suspected dealing ring
The six arrested include two senior city professionals at leading city institutions and one city professional at a hedge fund.
They are suspected of being involved in what the FSA calls "a sophisticated and long-running insider dealing ring".
It is alleged the city professionals passed inside information to traders, either directly or via middlemen.
The traders then allegedly acted on this information and made significant profits as a result.
A total of 143 FSA employees took part in the operation, together with officers from Soca.
"There can no longer be any doubt that the FSA is serious about cracking down on City crime, especially illicit trading in shares and securities when in possession of privileged insider knowledge," said BBC business editor Robert Peston.