Travelers over the age of 16 arriving from another state or country must submit the form online prior to or upon arrival at Los Angeles International Airport, Van Nuys Airport or Union Station.
"To help stop the spread of COVID-19, Los Angeles is requiring all travelers entering the city from another state or country to fill out this online form, acknowledging that they have read and understood California's travel advisory," the form states on the city website.
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The form is available at travel.lacity.org. Those who don't submit the form may face a fine of up to $500.
"Visitors arriving at LAX or Van Nuys will be informed of this new requirement through signs posted throughout the airports and airlines are being asked to notify their passengers," the mayor said during the coronavirus briefing. "It will take less than a minute."
Flightradar24, which shows real-time commercial aircraft flight tracking information on a map, recorded heavy air travel over the United States on Monday from 6 a.m. to noon.
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Despite warnings from health officials, more than 2 million Americans are projected to fly for Thanksgiving. On Sunday, airports saw their busiest day since the pandemic started, and ABC News reports more than 1 million people passed through American airports on Friday -- 1.5 million less than on the same day last year. However, the figure from the TSA represents the second-highest number of people to travel by air since March 17.
AAA predicts 50 million people will travel for Thanksgiving, 95% of which is expected to be travel by car. In Southern California, the majority of the 3.9 million people expected to travel over Thanksgiving will be driving.
The announcement came as Garcetti warned if the current pace of COVID-19 cases and hospitalizations continues, there will not be enough hospital beds by Christmastime. He added that the virus is "threatening to spiral out of control" in Los Angeles.
RELATED: LA Mayor Garcetti calls COVID situation 'very dangerous,' urges Angelenos to help stop spread
"We are at the highest risk level that we have been," he said. "We're seeing numbers that literally three weeks ago were half as much."
L.A. County shattered its previous daily case record on Monday, reporting 6,124 new cases.
Garcetti also announced the city's testing sites will be closed Thursday and Friday due to the Thanksgiving holiday.
City News Service contributed to this report.