The thousands who attended Boston Calling, this weekend’s two-day music festival at City Hall Plaza, went through toughened security measures on their way in today, including pat-downs.

Because of three recent fatal drug overdoses at concerts in Boston and New York organizers said security would look out for signs that patrons may be using Molly, a potent form of the club drug MDMA.

The first day of the festival was centered primarily on indie rock and folk acts, not commonly associated with Molly use, but dance and electronic acts dominate much of Sunday’s lineup.

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Michael Snow, co-founder of festival organizer Crash Line Productions, said he did not consider canceling the concert, but did ensure that security personnel paid attention to the crowds.

“Boston police and our security team are more attuned to look at people within the event. There are tell-tale signs,” Snow said. “But it’s a pill. It’s very hard to stop.”

Six Flags New England postponed a Sept. 28 electronica concert “in light of recent events.” In Manchester, N.H., the Verizon Wireless Arena canceled the local stop of the Barstool Blackout Tour, a traveling rave.

In Quincy, there were 12 reported drug overdoses at the Ocean Club in Marina Bay this summer, most of them involving Molly. Quincy police have arrested eight people for possession or distribution of the drug since Memorial Day.

Boston police spokeswoman Cheryl Fiandaca said the department will have more officers patrolling the Boston Calling concert this weekend than it did during the festival’s first show in late May.

“In light of the recent overdoses, additional trained undercover drug control officers will be also be deployed,” she said in an email.

At Boston Calling, concertgoers moved across the plaza between two stages as bands played back-to-back sets. Tonight was scheduled to conclude with two rock bands, The Gaslight Anthem and Vampire Weekend.

Tickets were still available for Sunday’s performances.

Bill Fulkerson, 36 of Lynn, sat on an elevated concrete vent as he watched the crowds and listened to the band Deer Tick.

“You’ve got the young kids with their vibrant hair [colors], and the hipsters with their pretty clothes and pretty hair,” he said. “I’ve seen some kids here with their families, which is pretty cool because it creates a nice overall environment.”