The Catalan club trailed for 90 minutes of the match before defender Ronald Araujo rose highest for a smartly-taken header in the dying embers of the clash.
The Uruguayan struck in the final minute of normal time amid a concerted flurry to cancel out Domingos Duarte's early opener and spare his side's blushes as the game finished 1-1.
The home crowd had been stunned into silence within two minutes of kick-off when Duarte headed Sergio Escudero's corner past keeper Marc-Andre ter Stegen and into the net.
Jorge Molina went close to a second for the visitors, but Sergi Roberto was denied at the other end by the crossbar and Memphis Depay failed to round off a quick break as the Catalan giants turned the screw.
Araujo, Sergino Dest and substitute Oscar Mingueza all went close with the half-time whistle approaching, but Barca went in at the break still trailing.
Araujo and Depay threatened again as the second half unfolded, but keeper Luis Maximiano and his defenders held firm with substitute Luuk De Jong heading just over from close range with 11 minutes remaining before Maximiano saved superbly from Araujo.
Substitute Gerard Pique was thrown into an attacking role as the home side fought desperately for a way back, and it duly arrived in the final minute of the game when Araujo headed them level in the nick of time.
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