From NPS.GOV (National Park Service,U.S. Department of the Interior)

Bear attacks are rare; most bears are only interested in protecting food, cubs, or their space. However, being mentally prepared can help you have the most effective reaction. Every situation is different, but below are guidelines on how brown bear attacks can differ from black bear attacks. Help protect others by reporting all bear incidents to a park ranger immediately. Above all, keep your distance from bears!

• Brown/Grizzly Bears: If you are attacked by a brown/grizzly bear, leave your pack on and PLAY DEAD. Lay flat on your stomach with your hands clasped behind your neck. Spread your legs to make it harder for the bear to turn you over. Remain still until the bear leaves the area. Fighting back usually increases the intensity of such attacks. However, if the attack persists, fight back vigorously. Use whatever you have at hand to hit the bear in the face.
• Black Bears: If you are attacked by a black bear, DO NOT PLAY DEAD. Try to escape to a secure place such as a car or building. If escape is not possible, try to fight back using any object available. Concentrate your kicks and blows on the bear’s face and muzzle.

If any bear attacks you in your tent, or stalks you and then attacks, do NOT play dead—fight back! This kind of attack is very rare, but can be serious because it often means the bear is looking for food and sees you as prey.

[Source: @exocomics]