LOOMIS, Calif.—At least 15 people suffered heat exhaustion and heat stroke at an outdoor graduation ceremony in Northern California, which saw record-breaking temperatures and conditions that posed extreme fire danger on Saturday, authorities said.

In the Placer County town of Loomis, approximately 7,000 people began filling Del Oro High School's stadium as early as 7:30 a.m. But within minutes of the start of the 9 a.m. ceremony, people began to collapse from the scorching heat.

"They were dropping like flies," the town's Fire Protection District Chief Dave Wheeler told the Loomis News. "We couldn't contain it and we didn't have the resources."

He said five people were hospitalized; two of them were initially listed in critical condition.

Wheeler said the combination of heat, people crowded onto metal bleachers, and the field's synthetic turf surface and little to no shade created a hazardous situation.

The National Weather Service said temperatures were 88 degrees there at the time. Wheeler said the field's artificial surface likely raised the ambient temperature by up to 20 degrees.

The injuries and extreme heat forced fire and school officials to cancel the event.

"We tried to mitigate it as much as we could," Wheeler said, at one point telling black-clad graduates to shed their robes as a safety precaution.

Wheeler said diplomas were handed for students with last names that began with A to D before the ceremony ended. The rest of the graduating class will have to pick up their diplomas at a later time or have them mailed.

The NWS said the mercury hit 112 degrees in Red Bluff, beating a record 108 degrees set in 1955. Chico and Marysville hit 110 degrees; downtown Sacramento and Stockton recorded 108 degrees; Modesto and Livermore reached 107 degrees.

The region saw a drastic drop in temperatures by as much as 20 degrees by late afternoon as marine air moved in, NWS meteorologist Drew Peterson said.

He said the cooling trend was expected to continue Sunday, with widespread thunderstorm, dry lightning and gusty winds which could pose extreme fire danger until late Monday.

As for the students, Wheeler said they had the weather on their side at their graduation party. It was held Saturday night at a water park.