Andre Shoukri, leader of the Ulster Defence Association in the north of the city, was held when officers swooped on a number of houses.
Three other men - at least one of them another senior UDA man - and a woman were also detained.
A senior security source confirmed: "This is an extremely significant operation."
It is understood that a decision to hold another person under the police's witness protection scheme was linked to today's arrests, which involved officers from the Police Service of Northern Ireland's organised crime branch.
Houses in the Westlands estate, north Belfast, as well as the Castlereagh area of east Belfast were searched.Reuse content