I lead a few activities with small groups of people, and I ran into this problem quite often. The way it happens usually the is "everybody starts talking at the same time".
I assume that there is not a big difference when only one person starts talking extra, or several persons do it.
Either way, I use this kind of reply:
Please wait with your answer a little, I cannot listen to two / several persons at the same time. Thank you.
An then I turn my attention to the person who was talking first. After that, I will give everybody around the chance to express their ideas, until everybody is satisfied.
NOTE: Of course, people will always want to say something, especially if it is a larger group, and the activity is more entertainment-oriented. In those cases, there is an ongoing effort to maintain the priority of taking turns talking. But with well-intentioned people, that is not a big deal.
There is especially one girl, she always needs to say something. Most of the times, totally unrelated to what goes on - even though she is a nice girl otherwise. She gives me most of the opportunities to train my moderation skills, and my patience, at the same time :)
It might happen that the person interrupting seems to have something important / urgent to say. In that case I address the person already speaking with something like:
Please wait a second, this seems important. We shall continue in a few seconds. Thank you.
And then I turn my attention to the new person, trying to keep the discussion to a minimum. Then I go back to the first speaker.
Note: I do not judge the importance of the message based on the excitement of the person. I evaluate his entire body attitude, and a very important factor is if the person was present to the discussion previously, or just arrived.
It might happen that somebody starts talking while I am speaking. In this case, the sentence / request changes a little:
Please let me finish my idea, and I will let you present your immediately afterwards.
Of course, there are cases when the person who interrupts does not get the point, and keeps doing that. In that case, I change the strategy a little:
- I start telling what I want to say.
- when they start peaking, I just adopt a "waiting" body attitude;
- when they finish talking, I just resume what I started to say in he first place.
- repeat the three previous steps until the person gives up, or until I finish my idea - whichever comes first;
With the above "strategies" I almost never needed to get to the point to become explicit:
Please do not be rude interrupting me. Use your education to refrain yourself talking what someone else already tells something.