Shops, banks and offices in Watford town centre have lost power.
The cause of the problem is still being investigated but it is believed a circuit board in The Harlequin shopping centre may have blown.
Several sets of traffic lights around the High Street are out, and several stores in The Harlequin shopping centre have lost power.
Kings and Queens carparks are also currently closed.
Mike Stevens, manager of The Harlequin, said only some stores were affected. He said about one third of stores, primarily the ones near John Lewis, have lost power.
Boots, Waterstones, WH Smith, Virgin and Game have all been affected.
McDonalds in the High Street has also shut.
He said: "We have been told it may be another 20 or 30 minutes until we have power again. Many stores are up and running, no problem. However some, even if they do have enough light, rely on Eftpos technology so they are currently not able to trade."
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