CARACAS—Most of Venezuela was left in the dark Monday in the latest blackout to hit the beleaguered nation as it suffers from an economic collapse and a crumbling power grid.

Local media reported that power was out in at least 19 of Venezuela’s 23 states since around 4:45 p.m. local time. Information Minister Jorge Rodriguez said in a brief televised address that the blackout was caused by an “electromagnetic attack” but didn’t provide evidence or further details. He said repair efforts were under way.

The...