A method of force estimation for a minimally invasive medical system including a robot manipulator having an effector unit equipped with a 6-degrees-of-freedom force/torque sensor and being configured to hold a minimally invasive instrument having a first end mounted to the effector unit and a second end located beyond an external fulcrum that limits the instrument in motion, where the method includes determining a position of the instrument relative to the fulcrum, measuring by means of the 6 degrees of freedom force/torque sensor a force and a torque exerted onto the effector unit by the first end of the instrument, and calculating by means of the principle of superposition an estimate of a force exerted onto the second end of the instrument based on the determined position, the measured force and the measured torque.