- A theoretical study is presented of electromagnetic plasma instabilities arising during the trapping of flare-accelerated protons by coronal magnetic fields of active regions. When beta (the ratio of the total pressure of plasma and energetic particles to the magnetic-field pressure) is greater than 0.1-0.3, the magnetic trap breaks down and the protons escape into interplanetary space. When beta is less than 0.1-0.3, the trapped protons excite cyclotron instability as well as the gradient instability of Alfven waves. The conditions for the strong influence of the cyclotron instability on the confinement time of protons in the trap are determined. In addition, it is shown that the gradient instability leads to the development of strong Alfven turbulence with small wavelengths in the direction across the magnetic field.