David Mooney looked to have given the home side the points and with three minutes remaining the O's were eyeing the chance to return to the top of League One, only for Posh to snatch two goals in three minutes.
Assombalonga rose to head home a Danny Swanson free-kick in the 87th minute and then two minutes later a 20-yard screamer from Rowe flew into the net to break the Orient hearts.
Mooney had taken his tally for the season to 17 goals in all competitions when he reacted quickest in the 70th minute to meet a Lloyd James free-kick and send a superbly placed header into the net.
Industrious midfielder Dean Cox came closest to breaking the first-half deadlock eight minutes before the break when his first time right-footed volley from a Scott Cuthbert cross thundered against the upright.
Three minutes later, O's leading striker Mooney saw his left-footed strike from close range curl narrowly past the post.
Posh also had their moments before the break with Nicky Ajose bringing a good save form Orient's debutant keeper Shwan Jalal. The striker also had claims for a penalty turned down when his shot struck the hand of Elliot Omozusi but referee Lee Probert was unimpressed
Orient had Jalal to thank for keeping them on level terms in the 62nd minute when he pushed a shot from Assombalonga onto the post and safety but it was the visitors who had the final word.
- Sports & Recreation
- David Mooney