Answer: "I am sorry that I won't be able to say everything I'd planned in my remaining time, but please let me conclude with . . ." Then state your most important idea and make your conclusion before time runs out.
Explanation: If you may be cut off by the moderator or the chairman, your audience will at least hear the conclusion you planned. If you are stopped ( and it DOES happen ) in mid-sentence or without making your most important point, the audience will see you as disorganized, or insensitive to the time limits, and may have an unfavorable impression of you-- and they will have missed the purpose of your speech.