DENVER – Gunfire erupted at a Denver park Saturday, injuring two people and sending tens of thousands gathered for an annual pot celebration fleeing, police said.

A crowd of marijuana smokers expected to swell to 80,000 had gathered at the park to mark the counterculture holiday known as 4/20 on the first celebration since Colorado and Washington made pot legal for recreational use. The shooting happened about 5 p.m., shortly after pot smokers shared hugs and joints in a mass 4:20 p.m. smoke-out.

Police spokesman Sonny Jackson confirmed two people had been shot and both were taken to a hospital with injuries that did not appear to be life-threating. The gunshots quickly dispelled the festive atmosphere, with police swarming the scene.

Witnesses said they heard three or more shots and crime tape was around the pavilion where the celebration was being held.

Aerial footage showed the massive crowd frantically running from the park.

A sizable police force on motorcycles and horses had been watching the celebration. But officers didn’t arrest people for smoking in public, which is still illegal.

Authorities generally look the other way at public pot smoking here on April 20. Police said before the event that they were focused on crowd security in light of attacks that killed three at the finish line of the Boston Marathon.

“We’re aware of the events in Boston,” said Denver police spokesman Aaron Kafer, who declined to give specifics about security measures being taken. “Our message to the public is that, if you see something, say something.”