Los Angeles County sheriff's deputies say they responded to a call of a possible cardiac arrest.
Junior running back Dodi Soza was in intensive care at St. Francis Medical Center. He is expected to be moved to Long Beach Memorial Medical Center to see a specialist.
His family isn't releasing his official condition, but friends describe it as very serious. Fans watching the game were stunned.
"Everybody just stopped. They all got in a group and started crying and I guess praying," said witness Saul Limones.
Teammates joined Soza's family at the hospital. On Twitter, #PrayForDodi was trending Thursday night.
"He's very well known around the school. He's a great running back. He's a good friend," said teammate Mario Galvan.
Soza's teammates are urging classmates on Twitter to wear white to school Friday in Soza's honor.
Lynwood Unified Superintendent Paul Gothold released the following statement:
"Our thoughts and prayers go out to the Downey High School player that collapsed during the freshman football game against Lynwood High School this afternoon, and his family. Following his collapse our team followed emergency protocol and immediately called the appropriate authorities to respond."