Partizan close in on Serbian league title
Partizan have 65 points from 27 matches, 16 more than closest rivals Vojvodina Novi Sad, and can win their 21st league title with five games to spare if they beat OFK Belgrade at home on Sunday.
Former European Cup winners Red Star Belgrade are out of the title race after Wednesday's 1-0 defeat at Javor Ivanjica left them 20 points off the pace.
Partizan missed several good chances before 17-year-old Ljajic, on loan at the club after signing for English and European champions Manchester United in January, broke the deadlock with a screamer from 25 metres.
The league's top scorer Diarra then side-footed home his 17th goal of the season after a one-two with Almami Moreira left the home team's defence flat-footed.
Partizan goalkeeper keeper Mladen Bozovic kept a ninth successive clean sheet to set a league record of 864 minutes without conceding a goal, surpassing the club's former stalwart Ivica Kralj.