The US presidential campaign has received a huge jolt of drama after Hillary Clinton left early from a 9/11 memorial event after becoming unwell - triggering fresh speculation about her health. Video footage showed the former secretary of state apparenty sagging and buckling as she waited for her vehicle and had to be helped inside.

In a move that was quickly seized on by opponents of Ms Clinton who have sought to try question her health, the Democratic candidate left the event at Ground Zero in New York after 90 minutes and went to her daughter’s apartment to recover. As she waited, propped against a cement pillar, she appeared to wobble and stagger. Many people rushed to help her into the black van and was then driven away.

Her spokesman, Nick Merrill, said that she left after feeling overheated at the event to mark the 15th anniversary of the attacks on New York and Washington, where she had gone to pay her respect to some of the families who lost loved ones. Almost 3,000 people died in the Al-Qaeda operation.

Clinton leaves apartment after 'episode' at 9/11 memorial

It was hot and humid in the city and Ms Clinton was dressed in a dark jacket and trousers and she was standing for much of the event.

“During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better,” Mr Merrill said in a statement.

US media said that Ms Clinton, 68, was taken to Chelsea Clinton’s apartment in the Flatiron neighborhood. Shortly before noon, Ms Clinton was seen leaving the flat and waving to the crowd.

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She briefly posed for a photograph with a young girl. To a shouted question from a reporter as to whether she felt better, she said: “Yes, thank you.”

She added: “It’s a beautiful day in New York.”

Her campaign said she was heading to her home to Chappaqua, in upstate New York.

The news was first reported by Fox News, one of the outlets that has been a platform for claims by 70-year-old Donald Trump that Ms Clinton has been hiding a medical condition from the public. One anchor, Sean Hannity, has interviewed various doctors, none neurologists, who have suggested Ms Clinton could be suffering from any of number of issues. The Clinton campaign has repeatedly denied the suggestions.

Fox News said that Ms Clinton had left the memorial and lost a shoe as she entered her Secret Service vehicle. It said that a police source at the scene, said a member of her security detail had to recover the shoe.

The Associated Press said a senior law enforcement official who was briefed on the matter said that after leaving the memorial plaza, Clinton was observed “fainting” in a departure area. Ms Clinton's campaign did not refer to this in its statement, though video of her at the scene showed Ms Clinton looking unsteady.

Ms Clinton has insisted she is in fine health and ready to assume duties in the White House. Her campaign has played down health scares, including an incident in December 2012 when she suffered a concussion and shortly afterward developed a blood clot.

In a letter released by her doctor in July, Ms Clinton was described as being in “excellent health” and “fit to serve” in the White House.