Credit: Zdenek Gazda

A frail-looking Hillary Clinton collapsed during the 9/11 memorial service in lower Manhattan on Sunday morning — prompting Secret Service agents to grab her arms to prop her up and help her into a campaign van.

The 68-year-old Democratic nominee appeared to fall ill while standing amid other politicians shortly before a moment of silence at 9:58 a.m., sources told The Post.

Videos posted online showed a wobbly Clinton leaning against a metal bollard on the perimeter of the memorial plaza. Secret Service agents grabbed hold of both her arms as she swayed and stumbled off the sidewalk along West Street.

The video shows Clinton’s head bobbling as she pitched forward before being lifted into the van.

The former secretary of state’s knees buckled, and she lost a shoe as her security team hustled her away, another witness said.

“The crowd started swaying a little, and everyone started talking,” said one source who was near Clinton in the crowd.

“A guy came up to a Secret Service agent and said, ‘She almost fell out. We’re getting her out of here.’”

The source added, “She looked like she got up at 3 in the morning and put on whatever was there. She wasn’t done up at all.”

Clinton — whose doctor later revealed she had pneumonia — was examined in the van by a Secret Service paramedic, said a law-enforcement source, who noted the situation was serious enough to leave her lost shoe behind.

The Secret Service apparently broke protocol by letting her wait for her ride, possibly because the situation was unexpected and unfolded quickly, The Washington Post reported.

It is a potentially damaging image for Clinton, with Donald Trump having repeatedly accused her of lacking the ‘strength” and “stamina” to be president.

Trump supporters, most notably former Mayor Rudy Giuliani, have gone further, accusing her of hiding an illness.

Before falling ill Sunday, Clinton was photographed looking pale and drawn while surrounded by pols, including Mayor de Blasio, Gov. Cuomo and Sen. Charles Schumer (D-NY).

Clinton was the only person in the crowd wearing sunglasses during the overcast morning.

In a statement issued around 11 a.m, about an hour after she abruptly left, campaign spokesman Nick Merrill said Clinton “felt overheated so departed to go to her daughter’s apartment and is feeling much better.”

The National Weather Service recorded the Central Park temperature at 79 degrees, with a heat index of 80 degrees, at around the time Clinton was stricken.

Clinton emerged from daughter Chelsea’s apartment near Madison Square Park at 11:45 a.m. and waved to an assemblage of reporters and onlookers.

Asked by The Post what had happened, she replied: “I’m feeling great. Thanks.”

“It’s a beautiful day in New York,” she added, bizarrely, as thousands of people were still downtown to honor the victims killed in the 2001 terror attacks.

Before being driven away in a van, Clinton hugged a girl who ran up to her on the sidewalk.

She arrived at her home in Chappaqua, about an hour away, at around 1:10 p.m.

About four hours later, her personal physician, Dr. Lisa Bardack, issued a statement saying that Clinton had been diagnosed with pneumonia Friday following a “prolonged,” allergy-related cough.

“She was put on antibiotics and advised to rest and modify her schedule,” Bardack said.

“While at this morning’s event, she became overheated and dehydrated. I have just examined her, and she is now rehydrated and recovering nicely.”

Last week, Clinton suffered a coughing fit during a Labor Day campaign speech in Cleveland.

She blamed it on seasonal allergies, telling reporters: “I just upped my antihistamine to try to break through it. It lasts a couple of days, and then it disappears. The advice, of course, is just don’t talk for a day or two, but that’s not going to happen.”

Trump was uncharacteristically quiet about his presidential rival’s health scare.

But in a well-timed twist, the Republican nominee and his eldest daughter, Ivanka, are set to appear on TV’s “The Dr. Oz Show” on Thursday.

“He addresses why the health of the candidates has become such a serious issue in this campaign, plus Mr. Trump reveals his own personal health regimen,” a promotional statement reads.

Additional reporting by Marisa Schultz, Kevin Sheehan and Reuven Fenton