"A hooligan cannot be allowed to blacken the name of the fans of @VillarrealCF," the police said on their official Twitter feed (@policia).
"We are looking for the person responsible for this act of madness," they added, posting a You Tube link with footage of the incident and including a telephone number the public can use to call in information.
Celta were leading 1-0 at the Madrigal and there were about three minutes left when the canister landed near the visitors' goal and began spewing out a thick cloud of white smoke.
The players, some of whom were clearly suffering the effects of the gas, retreated inside the stadium and fans quickly left the ground, with many covering their mouths and noses and in obvious distress.
The match was eventually restarted after about half an hour later in an almost empty arena and Celta forward Nolito made it 2-0 from a free kick in the 90th minute.
(Reporting by Iain Rogers, editing by Ed Osmond)
