A stage cementing tool has a mandrel defining a central flow passage and a plurality of cementing passages in a wall thereof. A packer is disposed about the mandrel. A closing sleeve is detachably connected to and movable relative to the mandrel. An opening sleeve defining a plurality of fully open cement flow ports therethrough is telescopically movable relative to the closing sleeve and movable relative to the mandrel. The opening sleeve is movable to move the packer to the set position, and thereafter movable to open a cement flow path between the central flow passage and an annulus defined by the stage cementing tool and the wellbore.