A system for assembly of a lifeline of the type that uses an anchor post and retractable drum assembly to horizontally stretch the mentioned as a means to protect workers constructing buildings or other structures from falls; the anchor post is modular and can be coupled to complementary tube(s) with telescopic adjustment means; said modular anchor post being provided with (i) a structural base with a plane region and ribs welded to the side of the post so as to keep said plane slightly removed from said anchor post; a hole drilled in the ground of the building receives the tubular sector, arranged between the end and the plane region; (ii) by at least one safety guide, formed by three arched segments; (iii) by at least one transport handle in a single piece; and (iv) by at least two brackets for mounting and attaching the retractable lifeline drum assembly, optimizing the ways of holding the lifeline stretched horizontally.