Muslim separatists cut off power supplies to at least 18 million people in the southern Philippines by blowing up electricity pylons, authorities said.
Troops stepped up security at Catholic sites
The Philippine army blamed the Moro Islamic Liberation Front (MILF), but a MILF spokesman said the group was not to blame.
The attacks happened on the island of Mindanao, where there has been heavy fighting between the rebels and Philippine government forces.
Electricity had been restored to about 90% of the island by mid-afternoon on Thursday, officials said.
The blackout, which started overnight, came as Philippine troops were on alert for what the military said were possible attacks by the MILF, the country's largest Muslim rebel group.
MILF spokesman Mohaqher Iqbal, interviewed on Catholic radio station DXMS, denied his group sabotaged the power supplies.
But he said the group was on a renewed offensive against the government, two weeks after the military captured a MILF stronghold on Mindanao.
The group has been fighting for a separate Muslim state in the south of the mainly Catholic Philippines for three decades.
On Wednesday the group said it wanted to end the violence but would only talk to Malaysian mediators until the government withdrew its troops from the captured rebel stronghold, in Cotabato province.
"The MILF has never abandoned a peaceful solution to the problem in Mindanao," MILF vice-chairman for political affairs Ghazali Jaafar told local television.
President Gloria Arroyo has approved the draft of a peace plan, which Malaysia is brokering.
The plans calls for a permanent truce and the disarming of MILF rebels, coupled with an amnesty and promises of political and economic reform.
A ceasefire was signed with the rebels in 2001, but sporadic violence has severely tested it.
The arrival of 200 US special forces, who have started 10 months of counter-terrorism training for Philippine troops in the city of Zamboanga, has also raised tensions.
The US troops are banned under the Philippines constitution from taking part in direct combat.
But some analysts believe the US will take a greater role than it did in similar exercises last year, when US soldiers were very close to the front lines.