In this situation, you're a presenter and you're getting bombarded with questions, is this right? Are people incessantly asking questions and not letting you answer, or do you just feel this way because you shut down in front of the audience?
First, relax, because as the presenter, you have the floor, and you should thus be in control of the room. This mentality has helped me in the past because if you're presenting, pending time limitations, you're the highest authority in the room aside from the chair/professor, and you're free to speak at your own pace, discuss things in your preferred order, etc. Others should respect this, and remembering this gives you power.
Pace yourself, and take time to think. If someone interrupts you, etc. tell them that you'd like to answer the previous question before you get to them. Stand tall, and crossing your arms behind your back helps posture to an extent, and try to not focus on one person in particular as you answer questions. Watching their reactions can be intimidating.
Just make sure to not focus on the people asking questions or how you feel about questions, but rather on how you feel about where you are in describing what you wanted to talk about.
Edit: If you don't have an answer, that is also fine, and never be afraid to say "I don't know". This is still part of the presentation, and that is what you have to offer. If you don't like not knowing it, ask the person asking the question if they can give you their information so you can follow up with an answer later.