In a frenzied first period Dortmund came from a goal down to lead 5-2 with Shinji Kagawa scoring twice and Marco Reus, back from a six-month injury break, also on target along with Nuri Sahin and Ousmane Dembele.
Legia's Aleksandar Prijovic netted twice for the Poles and also hit the woodwork in a topsy-turvy first half.
Dortmund coach Thomas Tuchel fielded a largely second-string team following their win over Bayern Munich in the Bundesliga on Saturday, including top striker Pierre-Emerick Aubameyang.
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Dortmund are top of Group F, two points ahead of Real Madrid, with their final group match in Spain in December.
(Reporting by Karolos Grohmann; editing by Ken Ferris)
