back from the deadball line for Song to score a rare goal.
Barca continued to dominate the game but Zaragoza looked dangerous when they did attack with Victor Rodriguez and Montanes going close with shots from distance before the break.
The home side then dropped their level with passes going astray, giving some hope to Zaragoza on the counter-attack.
But Messi responded to wrap the game up with half an hour to go with another clinical finish. This time he was found by the right full-back Montoya and he found the bottom corner from the edge of the area.
Earlier, a contentious late penalty by Roberto Soldado condemned nine-man Espanyol to a 2-1 defeat by Valencia.
Bottom-side Espanyol more than held their own after Samuele Longo equalised Jonathan Viera's opener but minutes from the end Hector Moreno was adjudged to have handled in the area. Replays suggest the ball hit his chest.
Moreno and Sergio Garcia were dismissed for complaining.
Elsewhere, Malaga drew 0-0 away to Osasuna.