Democratic White House hopeful Hillary Clinton cancelled a California campaign fundraising trip after she fell ill at a 9/11 memorial ceremony Sunday and her doctor revealed she was diagnosed with pneumonia.

"Secretary Clinton will not be traveling to California tomorrow or Tuesday," spokesman Nick Merrill said, hours after the 68-year-old candidate abruptly left the Ground Zero memorial suffering from what her doctor called dehydration.

Clinton has maintained a busy schedule over the past six weeks, attending three dozen fundraising events in August.

Her schedule modification comes in the heat of a tightening race, and just two weeks before she and Republican rival Donald Trump square off in their high-pressure first presidential debate.

She was due to attend finance events in California on Monday and Tuesday.

There was no word from the campaign on whether she would go ahead with her plans to campaign in the Las Vegas area Wednesday, or whether she might do other campaign events closer to home Monday and Tuesday.

Trump -- who also attended the 9/11 ceremony -- was uncharacteristically silent on Twitter about Clinton's illness, as both took a break from formal campaigning to mark the somber day.

But the businessman, his spokespeople and surrogates have promoted the idea in recent weeks that Clinton has serious health problems.

Trump, 70, has said Clinton is "not strong enough to be president" and that she "lacks the mental and physical stamina to take on ISIS" jihadists.

The former secretary of state "has been experiencing a cough related to allergies. On Friday, during follow-up evaluation of her prolonged cough, she was diagnosed with pneumonia," her physician Lisa Bardack said in a statement Sunday released by the campaign, adding that Clinton became "overheated and dehydrated" at the 9/11 memorial.

"She was put on antibiotics, and advised to rest and modify her schedule."