A blackout forced evacuations at a number of Disneyland attractions in California on Wednesday, which was crowded on a day many were off for year-end holidays.

Visitors at "Toontown" and "Fantasyland" attractions were plunged into darkness, as was the monorail that runs through the park, local media said.

It was not immediately clear how long it might take to restore power fully at one of California's most popular attractions.

The park was near full, during the busy Christmas vacation season.

"We had an issue with one of our transformers. So we lost power to certain areas of Disneyland park earlier today," said Disney spokeswoman Suzi Brown.

"Power has been restored to Toontown and we still have some areas of the park that are without power.

"The monorail is one of the attractions that are down ...(but) the majority of the park is operating," she added.