Imran shared in a second-wicket stand of 105 with Wajahatullah Wasti, who was left unbeaten on 80 when bad light forced an early close, with Pakistan on 192 for 3 after Wasim Akram had won the toss and elected to bat first.
Wajahatullah, playing in only his second Test, has faced 187 balls so far and has struck 12 fours.
Pakistan recovered well after Pramodya Wickremasinghe had bowled Saeed Anwar with the third ball of the match.
Imran, who earned a Test call-up after a recent excellent performance against the South African Under-19 team, quickly showed his class with a string of powerful shots.
His 67-ball half-century was studded with eight fours and a six, off Chandika Huthursinghe. By contrast, Wajahatullah faced 121 balls before reaching his 50.
Imran fell soon after play resumed following a 138-minute stoppage for rain. He had added only five to his lunch score when he mis-timed a hook shot off Wickremasinghe and was caught at deep fine leg by Sajeeva de Silva. His 87-ball knock contained 10 fours and a six.
Wickremasinghe captured his second wicket in five balls by bowling the out-of-form Inzamam-ul-Haq for a duck with a fine inswinger. He finished the day with figures of 3 for 61.
Wickremasinghe, exploiting the heavy conditions, should have had a fourth wicket but Russell Arnold dropped Yousuf Yohanna at first slip when he had scored one.
It was to prove a costly mistake as he and Wajahatullah compiled an unbroken stand of 87 for the fourth wicket.
Pakistan, with 17 points, are already through to the 12 March final in Dhaka. India have 10 points and Sri Lanka four.
First day; Pakistan won toss
PAKISTAN - First Innings
Saeed Anwar b Wickramasinghe 0
Wajahatullah Wasti not out 80
Imran Nazir c de Silva b Wickramasinghe 64
Inzamam-ul-Haq b Wickramasinghe 0
Yousuf Youhana not out 40
Extras (2b, 3lb, 3nb) 8
Total (for 3, 65 overs) 192
Fall: 1-0, 2-105, 3-105.
To bat: Shahid Afridi, Moin Khan, *Wasim Akram, Saqlain Mushtaq, Shahid Nazir, Fazl-e-Akbar.
Bowling (to date): Wickramasinghe 19-7-61-3; De Silva 11-0-66-0; Hathurusingha 10-3-21-0; Bandarathilake 18-7-25-0; Kalpage 7-1-14-0.
Sri Lanka: R P Arnold, M S Atapattu, D P M Jayawardene, A Gunawardene, *H P Tillakaratne, R S Kaluwitharana, R S Kalpage, K S C de Silva, U C Hathurusingha, M R C N Bandarathilake, G P Wickramasinghe.
Umpires: R E Koertzen (SA) and D R Shepherd (Eng).Reuse content