I am looking for a word which describes the emotion that one experiences on helping someone (friend, colleague, family, stranger). This helping out is not in the context of charity, but more of as helping out a social/societal equal. Is there any such word ?
I don't believe such a single word exists.
There are psychological theories that propose that all acts of altruism are ultimately self-interested, because of the feelings of satisfaction that arise. For instance, see wikipedia's article on psychological eogism or altruism. If there were a single word that meant this feeling, it would undoubtedly be invoked in such a context -- but it's not.
Since I've never heard of such a word, and can find no evidence of it existing in fields where it would naturally arise, I conclude that no such word exists in English.
An "empathic throbbing"! That'll do it.
There are many one-word expressions which describes the action of doing something unselfishly for others but as starwed affirmed hardly any in expressing the actual feeling in performing a good deed. However, there are ways round this limitation other than suggested.
Thoughtful Having or showing heed for the well-being or happiness of others and a propensity for anticipating their needs or wishes.
Selfless Having, exhibiting, or motivated by no concern for oneself
Considerate Showing kindly regard for the feelings or circumstances of others
Altruism: Pure altruism consists of sacrificing something for someone other than the self (e.g. sacrificing time, energy or possessions) with no expectation of any compensation or benefits, either direct, or indirect (e.g., receiving recognition for the act of giving).
Gratified: experiencing or manifesting pleasure
To feel a warm glow: a warm feeling, as of pleasure or well-being.
A warm fuzzy feeling: informal, full of love and kindness
A glow of happiness: e.g. "Anne felt a glow of happiness as she held her new baby."
EDIT: I didn't see the condition you wrote about charity. So, to change my answer, you could just use the word 'love' to describe it.
'Relief' is a good word, too. (It's relieving to help someone, because you're relieving their problems, and it makes you feel better. To one degree or another, other people's problems are your problems, too—at least, if you like them. That doesn't mean you have to help them (whether or not you really should), but it does mean that their problems are going to be a stress in your life, unless you're good at compartmentalizing, or unless you use coping mechanisms or something. But, just nipping it in the bud and doing some service can go a long way to removing the stress altogether without the need for coping. The fact that there are homeless people on the streets is probably a stress to you one way or another. If you were able to do something to help them all find homes, I imagine you would feel a lot better, even though the stress might have seemed to have been a small one initially. I propose that we would be very happy in ways we haven't fathomed if everyone else's problems suddenly went away. What seems normal to us is potential for a whole lot of relief—potential we might not have realized. These are some thoughts, anyway, to help you understand some reasons why 'relief' could be a good fit.)
'Care' is also a good word.
I think 'compassion' might be the best word for this. The connotation according to the dictionary tends to be before the service, though, but I tend to think you can feel compassion during the service, too.
If you're Christian, some people in at least some contexts might describe it as 'the Spirit' (referring to feelings the Holy Ghost gives you when you do something good, or when something good happens).
Philanthropic fits fairly closely (e.g. feeling philanthropic), however, it can be used before and after the fact, too. It means, "of, relating to, or characterized by philanthropy". Philanthropy means, "1: goodwill to fellow members of the human race; especially : active effort to promote human welfare". The quotes are from merriam-webster.com.
A word to describe the emotion of helping someone in my opnion differs on the reason why you're helping them:
Helping someone find a destination when they are lost makes me feel useful and it is fulfilling to be able to help another along their way.
Helping someone get through a hard time in their life can be humbling, as well as calling forth empathy and sympathy.
Helping someone to move furniture from one house to another raises gratitude, that I have a home and that I'm physically healthy and able to help my friend with this thankless job.