Cetlic recovered from the loss of an away goal to take a first-leg lead against HJK Helsinki in Champions League qualifying round three.
Gary Hooper, James Forrest and Joe Ledley all had chances as the hosts started brightly.
But it was HJK's Rasmus Schuller who broke the deadlock with a close range volley just after half-time.
Hooper equalised with a tap-in before Charlie Mulgrew headed Celtic ahead in the tie.
Neil Lennon's men take on the Finns in next Wednesday's second leg at the Sonera Stadium.
The home crowd were buoyed by Celtic fan Michael Jamieson's Olympic silver medal success at London 2012, with the swimmer's 200m breaststroke final shown on large screens in the run-up to kick-off.
Schuller's goal stunned the home crowd
The Scottish Premier League champions threatened from the off with Georgios Samaras heading a long ball onto Hooper only for the angle to get the better of the striker.
Samaras was prominent again with a cross towards James Forrest, who headed wide before indicating his displeasure at Mikko Sumusalo's aerial challenge in the six-yard box.
Hooper and Kris Commons failed to connect properly with Forrest's cutback and Ledley flashed a shot wide from the loose ball before testing goalkeeper Ville Wallen with a controlled volley moments later.
HJK weathered the storm and Joel Perovuo forced Celtic keeper Fraser Forster into a save with a low 25-yard drive.
And the Finns dealt a blow to Celtic's hopes when a neat passing move put Schuller through on goal shortly after the break.
The midfielder's first effort was blocked by Forster but ricocheted back to Schuller off Victor Wanyama and the resulting left-foot volley found the net.
Celtic responded with Commons' powerful drive striking the post and rebounding out for a corner off the head of Wallen.
But the leveller did come when Samaras' low centre set up Hooper to finish.
The hosts were galvanised and Mulgrew's downward header from Commons' corner edged them ahead.
The two players involved in that second goal came close with shots as HJK came under continued pressure and Forrest collided with team-mate Hooper as he tried to get into a scoring position off substitute Anthony Stokes' pass.
Scott Brown was then denied by Wallen and Lennon introduced skilful winger Paddy McCourt in an attempt to force a third.
But HJK held firm against the late rally and the Finnish champions know a 1-0 next week in Helsinki will take them through to the play-off round.
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