Jersey marked 71 years since its liberation from Nazi occupation on 9 May with a service and re-enactment in Liberation Square.
Bailiff, William Bailhache, gave his speech twice as the power cut hit just before he was due to speak.
Heavy rain also caused a number of people in the audience to leave early.
Hundreds still turned up to mark the event, seen as the island's national day including ex-servicemen and occupation survivors.
Organisers had planned a change to the traditional order of service, moving the Bailiff's speech to before the religious portion of the programme.
As the power cut hit just before the Bailiff was due to speak he was asked to give his speech again, with amplification, during the religious portion, returning to the normal order.
Jersey Electricity say the power was off in both islands but did not say why. The power cut lasted about 20 minutes.
In his speech, William Bailhache urged people to be tolerant of other people's religion, describing Jersey as a tolerant society.
He said: "In this Island today let us be beacons of tolerance, respecting those of different backgrounds, ethnicity, gender, sexual orientation or religion."
A traditional part of the service is the singing of Jersey's unofficial anthem, Beautiful Jersey.
For decades it has been sung by Sadie Rennard, but for the first time student Rachel Hayden performed the song.
The re-enactment saw Jersey's army cadet force replicate the original liberating forces journey through a packed Liberation Square in 1946.
This was then followed by a parade of the Jersey Field Squadron, bands, cadet forces and classic vehicles.
Jersey's Liberation Day celebrations were hit by an island-wide power cut, causing the big screens and public address system to fail.