Nine Britons and three Irishmen are being held in Ibiza as part of a major police swoop against drug traffickers on the party island.
Huge batches of the type of ecstasy pill linked to the death of 20-year-old Briton Jodie Nieman were among substances seized in a string of raids.
Spanish officers were targeting a gang suspected of travelling from the UK to Ibiza in a bid to profit from the island's population of clubbers.
More than 69,000 euros, 4kg of cocaine, 3,600 ecstasy pills, 5kg of crystal MDMA and drug-making equipment were seized at properties in the resorts of Sant Josep de sa Talaia and five in San Antonio de Portmany.
Most of the pills found are known as Pink Rock Star, similar to those linked to the poisonings of several clubbers, Spanish police said.
Suspects from Liverpool, Manchester, Plymouth and Croydon, south London, were arrested with the three Irishmen.
The Serious Organised Crime Agency worked alongside the Spanish Guardia Civil to execute the arrests on Friday.
A total of 13 suspects, including one Pole, were arrested as eight properties were raided.
A Spanish police spokesman said: "The detainees are members of one of the most active gangs on the island which is the main supplier of cocaine and other designer drugs around the clubs and bars.
"Inquiries were carried out on the basis of intelligence obtained by the Guardia Civil after other gangs involved in drugs trafficking on the island were dismantled.
"The majority of these gangs were British and took advantage of the influx of young people during the summer.
"Inquiries found that the gang, which is now dismantled, only travelled to Ibiza in summer as to meet the large demand for drugs on the island during this period."
Ms Nieman, from Croydon, had been clubbing at the Space nightclub in the Playa d'en Bossa and was taken to the Can Misses hospital after suffering a heart attack in July.