Second-half goals from Jordan Owens and Paul Heatley ensured that Crusaders lead the Premiership table from rivals Linfield on goal difference.
Owens's 57th-minute header squirmed through the legs of home goalkeeper Billy Brennan and Heatley rolled the ball into the net in the final minute.
Colin Coates had given the Crues a sixth-minute lead, shooting home after a pinball session in a crowded area.
Darren Murray slid in to score from a Peter McMahon cross before the break.
The result condemned the Ports to a third straight league defeat and they remain seventh in the table.
Coates's opener followed a Michael Carvill free-kick but Sean Mackle squandered a good chance to equalise soon after, while Sean O'Neill saved well from Murray.
McMahon also fired wide of the post, before setting up Murray for the equaliser at the end of the opening half.
Owens restored the champions' advantage with his 11th goal of the season, then Heatley touched the ball past the keeper, before finding the net in added time, his 11th of the campaign also.