Cape Town - Seven key underworld figures have been arrested in Cape Town in connection with nightclub security.Sources on Friday told News24 the suspects had been taken into custody.News24 witnessed two of the five handcuffed and in the company of police officers.The seven are believed to be involved in a security racket which has been taking over club security from a more established grouping.Their identities are known to News24. However, they have not yet been officially charged and may therefore not yet be named.It is understood the arrests relate to a letter, signed by one of those detained, that is said to have been distributed among club owners around Cape Town.The arrests on Friday of seven suspects linked to underworld nightclub security is said to be linked to a letter distributed to club owners. (Picture: Supplied)The letter starts by thanking members of establishments."We thank you for your loyal support throughout this year and we hope to be in partnership for many more," it says.The letter says the days venues will be open need to be established so staff can be allocated to these."Please note that the service that was rendered for the year 2017, we would like to request that we charging you double for the Services Rendered," it ends.At least two of the seven detained on Friday have previously been arrested in connection with other matters.Western Cape police did not immediately respond to queries regarding the development.News24 understands club owners who have received the letter linked to the matter may contact police.For a detailed breakdown on what has been happening in the underworld, see News24's showcase Underworld Unmasked.