Hillary Clinton (second from right) leaving the ceremony in New York

Hillary Clinton was diagnosed with pneumonia on Friday, her doctor has revealed, after the Democratic presidential nominee abruptly left a September 11 anniversary ceremony feeling "overheated".

A video showed Ms Clinton slumping and being held up by three people as she was helped into a van after the event, and her doctor said in a statement that she had become overheated and dehydrated.

"I have just examined her and she is now rehydrated and recovering nicely," Lisa R Bardack said in a statement.

The physician said Ms Clinton has had an allergy-related cough, and during a follow-up examination on Friday she was diagnosed with pneumonia, put on antibiotics and advised to rest and modify her schedule.

Less than two months from election day, it was an unwanted image for Ms Clinton as she tries to project the strength and vigour needed for one of the world's most demanding jobs. Republican rival Donald Trump has spent months questioning her health, saying she does not have the stamina to be president.

Ms Clinton's departure from the event was not witnessed by the reporters who travel with her campaign and aides provided no information about why she left or her whereabouts for nearly two hours. Spokesman Nick Merrill eventually said she had gone to her daughter's nearby apartment, but refused to say whether the former secretary of state had required medical attention.

Ms Clinton left the apartment on her own shortly before noon. She waved to reporters and said: "I'm feeling great. It's a beautiful day in New York."

The video of Ms Clinton posted to social media shows her being held up by aides as a black van pulls up. She stumbles and appears to fall off the curb as she is helped to the vehicle.

After leaving her daughter's, Ms Clinton was driven to her home in Chappaqua, New York, and made no public appearances. She is scheduled to fly to California for fundraising and it is unclear if her schedule will change.

Mr Trump, who attended the same event marking the 15th anniversary of the 9/11 attacks, was noticeably restrained. Asked by a reporter about Ms Clinton's health incident, he said: "I don't know anything."

The incident compounds an already difficult stretch for Ms Clinton as the presidential race enters its final stretch. Despite Mr Trump's numerous missteps, the race remains close and many Americans view Ms Clinton as dishonest and untrustworthy.

On Friday, she told donors that "half" of Mr Trump's supporters are in a "basket of deplorables" - a comment that drew sharp criticism from Republicans. She later said she regretted applying that description to "half" of Mr Trump's backers, but stuck by her assertion that he has given a platform to "hateful views and voices."

Now Ms Clinton is sure to face new questions about whether she is physically fit for the presidency. Mr Trump and his supporters have been hinting at potential health issues for months, questioning her stamina when she takes routine days off the campaign trail and reviving questions about a concussion she sustained in December 2012 after fainting. Her doctor attributed that episode to a stomach virus and dehydration.

Ms Clinton's doctor reported she is fully recovered from the concussion, which led to temporary double vision and discovery of a blood clot in a vein in the space between her brain and skull. She also has experienced deep vein thrombosis, a clot usually in the leg, and takes a blood thinner to prevent new clots.

Ms Clinton spent about 90 minutes at the 9/11 event on Sunday, standing alongside numerous other dignitaries, including New York's Democratic senators Chuck Schumer and Kirstin Gillibrand. The weather was warm and humid in New York, and there was a breeze at the crowded memorial plaza during the ceremony.

AP

PA Media