BOSTON—More than 500,000 homes and businesses across Massachusetts remained without power following an October nor'easter, and it could be days before all are back on line.

The storm that dropped a foot of snow in some area late Saturday into Sunday forced dozens of school closures on Monday, and prompted some communities to either postpone or cancel Halloween trick or treating.

National Grid reported more than 350,000 customers still without power Monday, with central Massachusetts the hardest hit. Western Massachusetts Electric Co. still had about 117,000 without power with near blackout conditions in Agawam, Easthampton and Greenfield. NStar was down to 46,000 outages Monday, with the Framingham, Waltham and Walpole areas the hardest hit.

The state Emergency Management Agency said about 800 people spent the night in 38 shelters across the state.

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