Officers found 67 canisters of nitrous oxide and dozens of balloons in the street and on the sidewalk near Boston's Fenway Park on Friday.
A large group of people were filling the balloons with nitrous oxide -- also known as laughing gas -- and then inhaling the gas to achieve a high before a Phish concert, police said.
"People were sitting in the corner just sucking on a balloon," said Robert Stevenson of Quincy.
Stevenson said he was at the concert and witnessed people selling and using the substance in the streets and parking lots around the ballpark.
"They would have like four or five balloons in their hands, and every once in a while, you'd hear 'Pop, pop,' you know, because the balloons would burst," said Stevenson.
He said the people selling the nitrous oxide had spotters to watch for police.
"You could tell the cops were coming because you'll see guys running with tanks and stuff them under the cars and stuff like that," said Stevenson.
People around Fenway Monday said they were surprised to see just how many nitrous oxide canisters were confiscated and the number of balloons that littered the sidewalks.
"It's not normal," said Ashley Simmons of Boston. "How would they even get their hands on it anyway?"
"That's a strange thing to be in this area, especially where it's so patrolled, but you never know these days. The world's a crazy place," said Nick Botsolis of Brookline.
Boston Police have not said whether anyone was arrested.