I saw this movie at the cinema in 1979 when it was released. I was 14, therefore 4 years too young to see it (UK, "X Certificate), but a member of friend group with a gruff voice went to the ticket office).
Large cinema, packed out, lots of excited mumbling until the film started.
After the face-hugger jump shot, total and absolute silence. And the at the chest-burster scene... absolute chaos. Screams. Real screams, not happy roller coaster screams. A few people leaving, unable to take it.
And it got me. It hit me hard. I had deliberately avoided any description of the movie so had no expectations, and then this happened, ,right there in front of me. The gore, the realistic acting, the pain of John Hurt...oh my...
That set the scene for the rest of the movie and you knew they weren't fucking about.
It traumatized me for weeks. Superb movie.
Edited on request.