Yep, I think it sounds like night terrors too. Our DS1 did that - it tends to happen a few hours after they've gone to bed (not 3am kinda times). He wouldn't respond to us and was still like he was asleep - only sceaming. We found that they only thing that stopped the screaming, was to put on the Night garden DVD. OMG - as soon as he heard the theme music, he would go quiet. We would sit up and hold him on our lap to watch some and then put him back to bed.
Our MCHN at the time gave me a handout which said not to wake them up or get them out of bed and they should stop crying/screaming and go back to sleep. Pffft. That never happened for us. He would just go on and on and on. He just grew out of it. Luckily DS2 didn't go through this. It's pretty full on, isn't it?