What should I do when my child hit other people e.g. grandparents, his class friends?

I would do the same as Jamie. Depending on your child's level of understanding, it is a good idea to make them understand that it is not OK to hurt someone. Get him or her to explain to you how they are feeling and why they hit. This might help you understand what they are going through and deal with the situation better.

Children may turn to violence when they are unable to express their frustration in a proper way. You have to make him understand that hitting people is NOT tolerated under any circumstance and help him find other ways to express his anger. http://sg.theasianparent.com/stop-kids-from-fighting-and-hitting/2/

My 2 years old boy also did the same. I will reinforce to him that he cannot hit and hitting is wrong and painful. I also borrowed books for him to let him relate that hands are not for hitting and nowadays he stop hitting already.

thanks