The Daggers had to collect a positive result to stand any chance of survival but were well beaten after failing to take a glut of chances.
Dagenham were undone defensively as early as the 11th minute when Posh youngster Nathaniel Mendez-Laing scampered into the box and fired under the body of keeper Tony Roberts.
But the visitors regularly had opportunities at the other end as Darren Currie twice hit the woodwork and striker John Akinde was twice denied by home keeper Joe Lewis before the break.
Yet Posh doubled their lead in the 65th minute thanks to a glorious strike from Grant McCann, who lashed his 10th goal of the season into the roof of the net.
The Daggers continued to create chances of their own but George Boyd guided in a third goal on 82 minutes and former Daggers ace Craig Mackail-Smith added his 32nd goal of the season two minutes later before David Ball completed the rout for play-off bound Posh.