Any comment will break the tension. Being prepared is the key to rehearsed spontaneity. Before making a remark, pause and look up like you are searching for something to say. The audience will think you are creating the humor on the spot. All humor must apply to your speech.
Relevance reduces resistance.
When taking advantage of humor of the moment, be ready with a comeback line. Some jokes I do just for me. I would now like you to bow your heads and join with me in silent prayer for that last joke that just died. You lose respect and credibility when you offend someone in your audience. Listen through the ears of the audience.