An investigation is under way amid suspicions the site, near the former Hovis factory in Cale Lane, Aspull, was targeted for copper wiring.
Travellers are now camped on the old bread factory site
Fed-up neighbours say this is the second time the complex has been hit by intruders.
Seven weeks ago police had to be called when another travellers encampment was established there.
Man killed in late night crash on A6 Chorley Road near Blackrod
Driving ban for 27 months for Wigan woman caught three times over the limit
Wigan woman who assaulted three people on one day receives community order
Wigan's week in court
Wigan carer who stole thousands of pounds from three vulnerable people is jailed
Heavy-duty two-tonne concrete blocks were left along the Cale Lane boundary, after the first encounter, which also resulted in a power cut.
But the current incident followed fencing being torn down on the Withington Lane side of the factory.
Neighbour Eileen Lawless said: “I was just settling down to watch something on the television when the power went out. This has happened to us before, around seven weeks ago, when there was a break-in. We were told all the wires were pulled out last time.”
Another neighbour added: “There are a lot of people down here who live on their own, who are affected when this kind of thing happens. My neighbour is 87 and we had to call her son to look after her.”
A spokeswoman for Electricity North West Ltd confirmed supplies were affected by “third party damage” on Wednesday night.
Some householders saw power return by early morning and the majority had their supply restored by early afternoon. Police have pledged to step up patrols in the area.
A police spokesman said: “We’re investigating offences that have occurred at the scene and we have recovered some property including a forklift truck.”
A Hovis spokesman said: “A group of travellers gained illegal and unlawful access to our Wigan site. Prior to this we have been taking advice from Wigan planning enforcement officers and external security consultants to put a number of measures in place we believed would safeguard the site.
“This included 24-hour security, the placement of large concrete blocks in front of all gates and in front of all the wire fencing.
“Last night’s attack was a violent and illegal breach of the site - criminals used significant force and smashed through the concrete wall to gain access to the site, putting our on-site staff at considerable risk.”
The spokesman confirmed an eviction notice has been served on the travellers.