Sri Lanka deployed troops on Monday to guard electrical installations as authorities investigated whether sabotage was behind the island's worst blackout in 20 years.
President Maithripala Sirisena ordered stepped-up security a day after outages plunged the entire country into darkness for at least seven hours.
"The president has instructed that security forces be deployed to guard electricity installations," power and energy ministry spokesman Pathum Pasqual told AFP.
"In the meantime, we are restoring supplies after yesterday's breakdown."
Authorities are probing the cause of an explosion and fire at a main distribution centre outside the capital Colombo that caused the entire electricity grid to switch off automatically on Sunday afternoon.
Power and Energy Minister Ranjith Siyambalapitiya said they had not ruled out sabotage.
"We were investigating technical reasons for the previous breakdown, but now we have another blackout," the minister told reporters.
"This means we have to look at other possibilities too."
The state-run Ceylon Electricity Board worked through the night to repair the centre and restore power to most of Sri Lanka, although several suburbs of Colombo were still in darkness on Monday morning.
The hours-long blackout brought water shortages as pumping stations could not operate without electricity. Most businesses were forced to close and Internet and phone connections in many areas were disrupted.
Hospitals in the capital continued to function after switching to generators.
Tonnes of frozen food and dairy products were spoilt because there was no refrigeration. Small businesses said they were trying to hire generators to keep their establishments open.
"I tried to hire a small generator this morning, but I was told everything had ben snapped up," said Chaandana Wijesekera, who runs a mid-size tailoring business in Colombo.
"I will not be able to execute my orders in time."
It was the second major power failure in less than a month, following a three-hour disruption in late February, and the worst since May 1996 when the entire country was without electricity for four days.
Power blackouts occur sporadically in Sri Lanka.
Sunday's nationwide outage came as an investigation was under way into last month's embarrassing disruption during a visit by New Zealand Prime Minister John Key, who had described Sri Lanka as a shining light in Asia.