The Home Office-funded campaign was carried out by 39 forces in England and Wales. Alcohol was seized from a total of 5,143 youths, 25% of whom were aged 15 or under.
Of the 30% who divulged where they had obtained the alcoholic drinks, half said they had bought it from a shop.
Home Office minister Vernon Coaker said:" I want to send a strong signal to those few irresponsible retailers who sell to under-18s. They will be caught and they will be punished."
The crackdown was part of a campaign to combine tough enforcement of the law with effective alcohol eduction for children and their parents.
"I want to remind parents of their responsibility, and where poor parenting is identified as an issue I want to see greater use of parenting contracts. I also want to see greater sharing of information between police and local agencies about repeat offenders," he said.
The Association of Convenience Stores (ACS) welcomed the crackdown. Chief executive James Lowman said: "ACS supports this operation because at last police time has been dedicated to tackling underage drinkers themselves. We believe that measures such as these show underage drinkers that there are consequences to their actions, but we need more enforcement against them or those purchasing on their behalf."