See this massive flash light up Donetsk skyline in Ukraine – Nov 20, 2014

A large, bright flash was seen late Wednesday in the Eastern Ukrainian city of Donetsk. The flash lasted for a few seconds, illuminating the area near Ruptly’s live cam.

Fighting has been ongoing in Donetsk between pro-Russian fighters and Kiev government forces.

Is a power station destroyed, an EMP attack or maybe they have used a tactical nuke?

If it is EMP-like effects, it looks like the distant one went off first. This could've disabled a power plant or other equipment that provided power to the area in the video. The few remaining lights could be backup power sources Then shortly after, another EMP-producing explosion/device goes off much closer to the video camera, which disabled electronics in this area and is what took down this video feed (which seems to have been interrupted during this closer, second attack.)