Benfica coach Jorge Jesus moved Brazilian international David Luiz from central defence to the left wing but the switch backfired as Porto created three first-half goals from that flank.
Winger Silvestre Varela opened the scoring in the 12th minute with a first-time shot from near the penalty spot after a fine run by burly Brazilian forward Hulk.
Colombia striker Radamel Falcao then added two, both from passes by Argentine midfielder Fernando Belluschi.
Champions Benfica failed to threaten a comeback after the break and suffered another blow when their captain Luisao was sent off in the 65th minute for elbowing Fredy Guarin.
Hulk converted a penalty on 79 minutes after being fouled by Fabio Coentrao and completed the rout with a long-distance drive one minute before the final whistle.
Porto are on 28 points from 10 games with Benfica on 18. Madeira club Nacional climbed to third place with a 1-0 win at Pacos Ferreira.
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