The plasma control system (PCS) of a long-pulse tokamak must be able to handle multiple control tasks simultaneously, and must be capable of robust event handling with a limited set of actuators. For ITER, this is particularly challenging given the large number of actuator-conflicting control requirements. To deal with these issues, this work develops a task-based approach, where a plasma supervisory controller and an actuator manager make high-level decisions on how to handle the considered control tasks, using generic actuator resources and controllers. This simplifies the interface for operators and physicists since the generic control tasks (instead of controllers) can be directly defined from the general physics goals. This approach also allows one to decompose the PCS into a tokamak-dependent layer and a tokamak-agnostic layer. The developed scheme is first implemented and tested on TCV for simultaneous beta control, neoclassical tearing mode (NTM) control, central co-current drive, and H-mode control tasks. It is then applied to an ITER test scenario to prove its flexibility and applicability to systematically handle a large number of tasks and actuators.