US Democratic presidential candidate Hillary Clinton has cancelled a campaign trip to California after being diagnosed with pneumonia.

Mrs Clinton was taken ill on Sunday at a 9/11 memorial ceremony and was seen stumbling as she left the event early.

The candidate’s team initially said she was “overheated”, later revealing she had been diagnosed on Friday with pneumonia.

Her doctor said she was now re-hydrated and “recovering nicely”.

Mrs Clinton was due to leave for California on Monday morning for a two-day trip that included fundraisers, a speech on the economy, and an appearance on the Ellen DeGeneres Show.

Her personal physician, Dr Lisa Bardack, said: “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.”

On Sunday, video showed Mrs Clinton being supported by aides as she entered a van to leave the 9/11 ceremony after an hour and 30 minutes.

She was taken to her daughter’s flat in New York’s Flatiron building and re-emerged later on Sunday, telling reporters: “I’m feeling great. It’s a beautiful day in New York.”

She then left for her home in Chappaqua, New York.

Republican presidential candidate Donald Trump (C) attends a commemoration ceremony for the victims of the September 11 attacks at the National September 11 Memorial and Museum in New York, 11 SeptemberMr Trump also attended the ceremony

Mrs Clinton’s Republican opponents have queried her physical fitness. She suffered coughing fit last week at a campaign event in Cleveland, Ohio which fuelled speculation about her condition.

Last month, Dr Bardack said in a letter that the candidate was “in excellent health and fit to serve as president of the United States”. She made a full recovery from surgery she underwent in 2012 for a blood clot, the doctor added.

But her Republican rival Donald Trump has repeatedly suggested she is unfit, telling supporters last month she “lacks the mental and physical stamina” to serve as president.

The Clinton campaign has accused opponents of pushing a “deranged conspiracy about Clinton’s health”. Mrs Clinton is 68. Her Republican opponent Donald Trump is 70.

Mr Trump has not published his medical records. Instead he released a note, in which his doctor declared that he would be “the healthiest individual ever elected to the presidency”.

Mr Trump’s team has not yet made any comment on Mrs Clinton’s pneumonia diagnosis.

Analysis – Anthony Zurcher, BBC News, Washington

George HW Bush once vomited on a Japanese prime minister. His son fainted in the White House after choking on a pretzel. Franklin Roosevelt hid his serious health conditions, and John F Kennedy never spoke of his debilitating back condition.

The difference between these men and Hillary Clinton, however, is that her “overheating” episode – the apparent result of a case of pneumonia – comes in the home stretch of a presidential campaign where she seeks to tie Ronald Reagan as the oldest person ever elected to a first term as president.

Then there are the conspiracy theories about her health – some advanced by top Trump campaign surrogates – which will become more frenzied. The campaign’s decision to reveal Mrs Clinton’s pneumonia diagnosis only after her very visible struggles two days later will certainly complicate matters.

Donald Trump, age 70, is not clear of medical concerns of course. The only information on his health comes from a haphazardly written note from his doctor.

Sunday’s episode may keep Mrs Clinton off the campaign trail for a few days, then fade away. People who would otherwise have shrugged off her health rumours, however, are now watching her activity more closely.

What the US press said

Writing in the Washington Post, Todd C Frankel said Mrs Clinton had for years been discreet about her medical history and her discretion “made it difficult for voters to assess her complete health picture”.

NBC News reporters said the incident had “sparked renewed calls for both Clinton and Trump, who are 68 and 70, respectively, to reveal more about their health”.

Adam Nagourney, New York Times LA Bureau Chief, tweeted: “Is this hadn’t happened, would Clinton campaign have announced pneumonia? One reason why Clinton/Trump should release medical records.”

Vox News pointed out that the US has a rich history of presidential candidates being less than clear about their medical histories. “Looking back, we now know a number of past presidents and presidential candidates who have actually been much sicker than the public knew.”

What is walking pneumonia?

Mrs Clinton’s team say she is suffering with “walking pneumonia” – a less serious type of the lung infection which leaves patients feeling unwell but doesn’t usually require bed rest or hospitalisation.

Pneumonia is essentially an infection of the lungs which causes inflammation in the air sacs and fills them with fluid. Symptoms can include a cough, fever, fatigue, chills and shortness of breath.

Anyone can contract pneumonia, although smokers, older people, and sufferers of chronic lung diseases are at increased risk. There are two types – bacterial or viral. Bacterial pneumonia is common and easily treated with antibiotics.

Most people with so-called “walking pneumonia” can recover within a few days. Those with weak immune systems or existing conditions can take weeks to recover, and pneumonia can in some cases be fatal.