Fast, continuous clicking noise< unless key is turned to

As soon as my car door opened (with my key) I heard a fast, constant clicking noise. When I turned the key on, to start engine, Nothing happened Except clicking stopped). The engine didn't try to turn over, nothing worked (auto windows or door locks; brake pedal was stiff; key in lock when door opens warning; inside lights etc.), Lifted the hood; no corrosion around battery clamps, but we could hear (slightly) the clicking sound bear the battery. The only thing that happened when I turned the key to 'on', the needles for engine temp and gas gauge needle. It's Friday night and my mechanic is gone until Monday morning. Is that constant clicking going to cause the battery or engine to do something dangerous, like blow-up, catch fire, or...? Thank you, in advance, for any suggestions, warnings, emergency something that we can do, while waiting until Monday morning? I appreciate this website, we are both retired, and disabled, and need to brace ourselves in the event of a costar repair...(eeek!)...