that Clinton's knees appeared to buckle as she left the service.
Her doctor has since revealed that the politician is suffering from pneumonia Her doctor Lisa Bardack said in statement: "Secretary Clinton has been experiencing a cough related to allergies. On Friday, during follow up evaluation of her prolonged cough, she was diagnosed with pneumonia.
"She was put on antibiotics, and advised to rest and modify her schedule.
"While at this morning's event, she became overheated and dehydrated.
"I have just examined her and she is now re-hydrated and recovering nicely."
A spokesman for Clinton, Nick Merrill, said Clinton had attended the service for an hour and 30 minutes.
He said: "Secretary Clinton attended the September 11th Commemoration Ceremony for just an hour and thirty minutes this morning to pay her respects and greet some of the families of the fallen.
"During the ceremony , she felt overheated so departed to go to her daughter's apartment, and is feeling much better."
One journalist who has been following Clinton on the campaign trail tweeted that Clinton left the 9/11 ceremony with the press pool not told where she was going.
Fox News also reported that the former Secretary of State had to wait for her motorcade for several minutes as her security detail had not realised she would be leaving early.
Clinton arrived at the New York ceremony in the morning, wearing dark glasses, having released a statement saying it was important to honour the lives and "tremendous spirit" of the victims and responders of September 11, 2001.
She and opposing candidate Donald Trump both attended the morning service to mark 15 years on from the September 11 attacks, with Trump remaining after she had left.
Clinton has been plagued by speculation that she is not fit enough to serve as US President.
She was even jokingly challenged to show she could open a jar of pickle to prove she was in good health .
The candidate also had her pulse taken as part of the joke.
Democratic presidential candidate Hillary Clinton spoke with New York Mayor Bill de Blasio at the service Credit: Reuters Clinton stayed at the service for an hour and a half before leaving Credit: Reuters Clips of Clinton appearing to cough up green mucus also sent ripples through the presidential race.
Clinton had a coughing fit while speaking at a rally in Cleveland, Ohio, forced to take a moment to sip some water and recover.
Her opposing candidate Trump has used the incidents to claim that Clinton is too weak to serve as the US president, also saying she takes too many naps to properly serve.
He also said he had "no idea" what had happened to Clinton when he was asked about her health following the ceremony.
President Obama observed a silence before speaking at the Pentagon Credit: Getty Images Clinton emerged from her daughter Chelsea's home two hours later walking unaided and waving to reporters.
The Democratic candidate repeatedly said she "felt great" when asked by journalists but failed to answer questions about what exactly happened.
She also stopped to hug a child before walking out of shot.
The revelation is likely to add fuel to rumours around the candidates health which have been circulating for some time.
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