Courtesy a fellow redditor (reddit.com), I learned something bizarre today. When I think about it, it makes total sense. Bystander effect is simply the name given to crowd behavior in a tense situation. More information here
According to wiki page and real life cases shared by people on reddit, you are more likely to get assistance if you are with a single person than when you are in a busy marketplace. Its not like people do not want to help but they look for direction i.e. initiative by someone. According to wiki they may think they are less qualified to help or are afraid because of social shame (what?).
I personally think any help is good help when someone in front of you is stabbed but well this is human nature and I may not be as much in control when I am in front of my computer as compared to in a place full of strangers watching something that I have only seen in movies.
So in scenario like this, according to studies people tend to take action by taking clues from others present there. Problem is others also trying the same and situation is sort of a deadlock in computer science terms (ohh, couldn't stop pointing this out :D).
So remember, in such a situation instead of asking for help from a crowd, single out someone and ask for help. This way that person will feel responsible for the situation and is more likely to assist you out or direct onlookers present on the scene on what to do next.