Detectives believe the 15 suspected con artists were cold-calling thousands of UK residents and pressurising them into parting with their savings for non-existent shares.
The men - aged between 22 and 41 - were arrested earlier this week in an armed raid by Spanish and British authorities on their suspected headquarters in Palma, Mallorca.
Detectives leading the investigation said they were living "lives of luxury in the sun" from the scheme.
Officers appealed for potential victims to come forward. Dyno Medical, Inca Pacific Gold and Mining and Viking Gold Resources were named as fake companies linked to the gang.
Detective Superintendent Bob Wishart, from the City of London Police's Economic Crime Directorate, hailed the raid as a "major result" in the fight against boiler room scams.
"It is the first time in Spain that authorities believe they have dismantled an operation of this scale in its entirety," he said.
He said gang members were talking on the phones as the raids took place. The officer added: "They were shocked when we went through the door. It was a real message for the crooks out there - we are very grateful for the work, support and co-operation we had from Spanish police."
The 15, indicted under Spanish law, are: Liam Rymell, 23; Dominic Jones, 24; Shafiq Dad, 41; Omar Rana, 27 and Rashid Shafayat, 30, all from Nottingham; Danny Dilliway, 25, from Canvey Island, Essex; Farhan Khan, 24, from Hayes, Middlesex; John Bartlett, 22, from Mansfield; Tyrone Robinson, 23, from Mansfield; Chris Savva, 30, from Grays, Essex; Lee Fisher, 24, from Mansfield; Mohamed Ghazalli, 26, also from Essex; Anthony Baugh, 25, from Luton; Neil Simpson, 27, from Basildon; and Fahim Khan, 36, from south London but residing in Spain. Two German nationals also appeared in court in connection with the alleged con.
They appeared briefly in a Spanish court on Thursday but are yet to face trial, City of London police said. Boiler room scams involve fraudsters using high pressure sales tactics to con investors into buying non-tradable, overpriced or even non-existent shares. They are thought to cost the UK around £200 million a year