Ten minutes in and Celta Vigo had the ball in the back of the net. The assistant referee's flag was raised, though, and the goal was ruled out.
The home side thought that they should have had a penalty shortly after. Park Chu-Young ran clear and seemed to be brought down by Marc Bartra but the referee gave the foul against the South Korean much to the anger of the crowd.
After regaining a measure of control, Barca were again under pressure midway through the half.
On 30 minutes Messi thought that he should have had a penalty of his own, when after beating two players on the right he cut into the box and seemed to be tripped. The referee waved away his claims.
Ten minutes later Celta were ahead. A great move involving Park, Natxo Insa and Fabian Orellana opened Barca up, and just when the chance seemed to have gone Insa popped up in the box to poke the ball in.
With just a few minutes to go until half time the away side equalised. Messi chipped the ball over the top for Cristian Tello, who broke the offside trap and finished well to ensure the teams went in level at the break.