Italian police have arrested 23 people accused of being part of a criminal network smuggling people across the Mediterranean, following testimony from a former gang member who said migrants had been beaten and killed en route to Europe.

The majority of the 23 were Eritrean and were detained in Rome, Palermo and the central city of Macerata. An Italian thought to be in charge of moving migrants from Rome to northern Europe was also among those arrested, and police were seeking 15 other suspects.

Investigators were acting on evidence from Nuredin Wehabrebi Atta, an Eritrean people smuggler who was arrested in 2014. Atta said migrants who could not pay for the journey to Europe had been beaten and in some cases killed by the smugglers or sold to gangs who traded in human organs.

“The cost of each man sold to an Egyptian criminal organisation involved in the selling of organs was about $15,000 [£11,300],” Atta told prosecutors.

A report released by Amnesty International on Friday based on testimony by survivors in Italy said migrants held in Libya suffered torture and rape and risked being killed by their captors. Some were sold on to criminal gangs on their arrival in Libya, Amnesty said, although the report did not detail a trade in organs.

As part of the latest Italian operation, police seized nearly €550,000 (£461,100) in cash found in a Rome perfume shop, said Palermo’s deputy chief prosecutor, Maurizio Scalia. Profits from people smuggling were reinvested in the drugs trade, Scalia said.

The sum would be small change for people smugglers, who collectively made $5-6bn from transporting people to Europe in 2015, according to estimates from Europol, the EU’s police agency.

The gang in Italy made further profits by producing fake family reunification documents, charging migrants $10-15,000 for the paperwork that would help their relatives reach Europe, it was alleged.

As the arrests were made on Monday, a separate trial got under way in Palermo of an alleged people smuggler who was extradited to Sicily last month. Italian prosecutors and British authorities claim to have caught Medhanie Yehdego Mered, a 35-year-old Eritrean accused of smuggling thousands of compatriots across the Sahara desert and on to Europe.

But ever since news of the arrest broke there have been serious doubts over the man’s identity, and some of Mered’s victims say the wrong man has been detained. Family members of a 29-year-old former dairy worker, Medhanie Tesfamariam Berhe, say their relative had been accidentally arrested and has no connection to people smuggling.

Berhe’s lawyer, Michele Calantropo, said he had submitted testimony from two of Mered’s former victims who had travelled to Sicily last week to testify in court. “They are both saying that Italy and Britain have bungled the operation and arrested the wrong man,” Calantropo told the Guardian.

The case was adjourned on Monday until 21 July and the suspect will remain in custody until the next hearing.