Dutch police found an "underworld prison" with a torture chamber hidden inside seven shipping containers following a huge hack of a Europe-wide criminal phone system, officials said Tuesday.

The torture cell was kitted out with a dentist's chair along with gruesome equipment including pruning shears, a saw, scalpels and pliers, Dutch police said.

Six suspects were arrested after armed police raided a warehouse with the containers using information from a Franco-Dutch operation to infiltrate the EncroChat encrypted phone system.

"Six served as prison cells in which people could have been tied up and one other container had the sole purpose of being a torture chamber," said Andy Kraag, head of Dutch police's central investigations division.

A video supplied by police showed armed officers raiding the premises, situated near the southern Dutch city of Bergen op Zoom, south of Rotterdam.

The containers were soundproof and covered inside with tin foil in what police believe was a bid to make them invisible to thermal imaging cameras. All had handcuffs installed to the floor and the ceiling.

The kidnappings of the people destined for the cells were planned with "great precision", police said, involving several teams, weapons, and fake police clothes, vans, stop signs and bullet proof vests.

One container, referred to as the "treatment room" by criminals in tapped conversations was kitted out "with a dentist's chair and straps to the arm and feet rests," police said.

Police recovered pruning shears, loppers, a saw, scalpels, pliers, handcuffs, finger cuffs and masking tape, saying the implements were "to torture victims or in any case, to put them under pressure".

Dutch police chief Jannine van den Berg said the discovery of the cells came after French and Dutch police hacked the EncroChat network, which was announced last week along with the arrest of around 800 people around Europe.

"We already announced then that there was more to come. The discovery of this underworld prison is an example of this," she said.