The FSA has made countering boiler rooms a top priority
Six men and a woman have been arrested in the UK in a further crackdown by the authorities against illegal "boiler room" share selling operations.
The Financial Services Authority (FSA) said victims of the alleged fraud appeared to have lost £28m.
The arrests were part of an attack on fraudsters involving authorities across Europe as well as the UK.
The FSA said the arrests in the South of England were part of the biggest such operation it has yet conducted.
The seven were arrested in early morning raids on Wednesday on seven homes and one office in London, Hertfordshire, Surrey and West Sussex.
FSA officers were helped by the Crown Prosecution Service, City of London Police and Hertfordshire Police.
The wider investigation has involved authorities in several European countries, including the Maltese anti-money laundering department.
The problem of boiler rooms, many based overseas, is widespread. Participants tend to phone people directly to urge them to make worthless investments dressed up as attractive propositions.
The FSA says it is told of about 500 new ones each year, although some may be the same outfits masquerading under new identities.
The FSA recently revealed that it was contacted by about 3,000 boiler room victims each year, reporting an average loss of £20,000 each.
But it says this is clearly the "the tip of the iceberg" and estimates that there are probably about 30,000 victims in the UK each year, with a collective loss of between £150m and £300m.