What should be the immidiate treatment for sever bleeding from limb?

First, try to stop the bleeding with firm, direct pressure to the wound using a clean cloth (though if the situation is severe enough, using your bare hand is better than the person bleeding out.) You may have to keep steady pressure on for fifteen minutes.

If direct pressure fails to work after fifteen minutes, and you know where the vessel is is, apply pressure to the major blood vessel supplying the wounded limb. (For example, for severe bleeding in the upper leg one would apply pressure to the femoral artery for that leg, which runs close to the surface in the groin.)

If direct pressure and vessel pressure both fail, a tourniquet may be applied to the limb above the wound.

NEVER TIE A TOURNIQUET WITHOUT A QUICK-RELEASE METHOD. Tourniquets cut off almost ALL of the blood supply to a limb, and a knot that must be fumbled with or cut in order to restore blood flow can be the difference between saving a limb and losing it.

If you are using a strip of cloth for the tourniquet, tie it more or less loosely around the affected limb, above the wound and not directly over a joint. Then slide a pencil or a stick under the cloth and turn the pencil/stick to tighten the cloth around the limb, securing the pencil/stick in place with another length of cloth.

If using a rubber tourniquet band, tie a slip-knot such that the tourniquet may be released by simply tugging one of the ends.

A blood pressure cuff may also be used as a tourniquet if necessary.

NOTE: Leaving a tourniquet on too long can cause severe damage to the limb.

ALWAYS write the EXACT time the tourniquet was applied DIRECTLY on the patient if possible, and in two places (ICUs and emergency rooms write it in indelible marker just above the tourniquet and on the patient's forehead!) Once a tourniquet has been applied, seek IMMEDIATE professional medical help by taking the patient to the nearest emergency room or by calling 911 (or the equivalent emergency number in your country) if the patient cannot be moved or if you suspect a spinal injury.