The present invention discloses a method for the occlusion of blood vessels or fluid vessels to stop the flow of blood or fluid during elective or emergency surgical procedures or following traumatic injury. This invention also encompasses a medical device or tool for occluding, reducing, or controlling blood flow or fluid flow. In this invention, a plug or blockade is formed to occlude, reduce, or control blood flow or fluid flow when magnetic microparticles, nanoparticles, or colloids are administered into a blood vessel or fluid vessel, and an external magnetic field source is placed at the desired site near or above the path of the blood stream or fluid flow. Removal of said external magnetic field source would result in the redispersion of the microparticle blockade and resumption of the flow.