New York's main contract, light sweet crude for delivery in January, slid 81 cents to $96.86 a barrel.
Brent North Sea crude delivery for January slipped 46 cents to $107.10 in London afternoon deals.
Crude was being pressured by "concerns about the European economies", said Victor Shum, an analyst at Purvin and Gertz energy consultants.
Spain's conservative Popular Party swept to victory in Sunday's general election and now has the task of pushing through severe austerity measures in the eurozone's fourth-biggest economy.
The Socialists are the latest in a series of scalps claimed by the region's debt crisis, which shows no signs of abating.
Governments in Portugal, Italy, Ireland, Greece and now Spain have been deposed as a result of the economic turmoil.
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