Nigeria's national power grid collapsed on Wednesday after a generator tripped, triggering a cascade of failures across the system, the state-backed Independent System Operator (NISO) said.

The outage occurred at 1020 GMT and caused a huge drop in load, NISO said in a statement.

The operator began restoring supply, starting with the capital Abuja from the nearby Shiroro hydropower plant, and said "substantial restoration" had been achieved nationwide.

Power grid collapses in Nigeria have often forced businesses and households to depend on diesel or petrol-powered generators to meet energy needs.