SAN FRANCISCO » At least a dozen people were arrested, most of them facing possible felony charges, during an annual, unsanctioned 4/20 marijuana celebration in San Francisco, police said Monday.

About nine people potentially face felonies, including two accused of possessing and carrying a loaded and concealed handgun during Sunday’s gathering at Hippie Hill in Golden Gate Park, said Sgt. Danielle Newman, a police spokeswoman.

There were also multiple arrests for possessing and selling marijuana or opiates, and numerous other citations, Newman said. Officers also responded to four medical calls, including a seizure and an overdose, she added. No deaths were reported.

City officials estimated that about 15,000 people attended the of the nation’s largest gatherings for the unofficial April 20 pot holiday, held coincidentally this year on Easter Sunday. With the past three 4/20 events held toward the weekend, crowds in San Francisco have swelled. The eastern end of Golden Gate Park was covered with canopies Sunday while scores of people smoked marijuana, and ate pot-laced cookies and brownies.

Last week, city leaders including Police Chief Greg Suhr and Supervisor London Breed warned revelers that police would be cracking down on extreme misbehavior. Last year’s event attracted about 15,000 people and had more than a dozen arrests, authorities said. It also left about 5 tons of trash that cost the city about $15,000 to clean up, San Francisco Parks officials and police said.

"The complaints last year were that there was a lot of fighting and unruliness," said Newman, who added that did not appear to be the case this year. "And of course the garbage, which we tried to prepare for as best as we could this year."

Crews and volunteer organizations began the big cleanup using special event bins shortly after the big "smoke-off" at 4:20 p.m. Sunday. Among the volunteers was the Green Cross medical dispensary, which said it collected some 350 bags.

"We wanted to help prevent what happened last year from happening again so there’s no backlash from future 4/20 celebrations," Holli Bert, a Green Cross spokeswoman, said Monday.

The trash collecting lasted well into the night, said Connie Chan, a parks spokeswoman. The cleanup was finished by midafternoon Monday.

"So far, from what we can tell, the park is in better condition than it was last year," said Chan, who added that the city’s Parks and Public Works departments will debrief with police later this week.