The national grid switched off early afternoon and attempts to reconnect failed as the system was unable to cope with night time demand, a spokesperson for the state-run Ceylon Electricity Board (CEB) utility said.
It was the second major power failure in less than a month, following a three-hour disruption late February.
Sri Lanka suffers hours-long power blackout
Six hours into the latest breakdown, there was no word on when power would be restored, making the latest blackout the longest since May 1996 when the entire country was without electricity for four days.
Sitting in the terrace. Islandwide power #blackout in Sri Lanka pic.twitter.com/47NRhA4Dm8
— މުހައްމަދު ޝާފިއު (@shafdawn) March 13, 2016
CEB Chief Anura Wijepala accepted responsibility for the latest breakdown and offered to resign, the state-run Daily News said.
Traffic signals failed and public transport was disrupted, while many shops were unable to process credit card payments and fuel pumps were out of action.
Karachi blackout: Power supply still interrupted in Shah Faisal Colony
Internet and mobile phone services were also disrupted in many places where there were no stand-by generators.
Power blackouts occur sporadically in Sri Lanka but the latest nationwide outage came even as an investigation was underway into last month's embarrassing disruption while New Zealand Prime Minister John Key was visiting the island.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ