Marcelo Bielsa's men, since taking one point from their opening two league outings this season, have now won eight consecutive games and hold a significant buffer over reigning champions Paris Saint-Germain and Bordeaux.
In Andre-Pierre Gignac, they have one of the form strikers in Europe and the 28-year-old netted his 10th goal in as many league appearances in 2014-15 to help his side to victory in front of a record crowd of 61,846 at the Stade Velodrome.
Marseille took the lead after 20 minutes when Nicolas N'Koulou headed home Dimitri Payet's free-kick into the top left-hand corner.
A fine team move led to the second, as Benjamin Mendy got free down the left and, when his low cross was not dealt with, Gignac rifled home the loose ball from 12 yards.
That victory put the pressure on Bordeaux to close the gap back to five points, but they were unable to respond as they were held 1-1 at home by struggling Caen.
Cheick Diabate's penalty midway through the first half put Bordeaux in front, but they were pegged back 13 minutes from time as Herve Bazile ensured a share of the spoils.
Elsewhere, Lyon moved up to fourth after making it six league matches unbeaten with a stunning 5-1 triumph over Montpellier.
Yoann Gourcuff scored twice and nabil Fekir was also on target before Siaka Tiene reduced the deficit.
But strikes from Alexandre Lacazette and Steed Malbranque inside the final eight minutes completed the rout.
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