"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 in New York suffering from dehydration.

The incident, in which a wobbly Clinton appeared to lose her footing as she was helped into her vehicle, offered Republican Donald Trump a new opening to attack his White House rival with just 15 days before their first high-pressure presidential debate.

Clinton had been seeking to bounce back from a blunder Friday, when she told donors that half of Trump's supporters belonged in a "basket of deplorables" -- so Sunday's episode was certainly ill-timed.

The former secretary of state spent 90 minutes at the ceremony in lower Manhattan, greeting some relatives of those killed in the terror strikes 15 years ago, her campaign said in a statement. Clinton was a US senator for New York at the time of the attacks.

"During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better," the statement said.

Later, the campaign released a statement from her personal doctor, Lisa Bardack, who revealed that Clinton had been diagnosed with pneumonia Friday and was suffering from dehydration.

"She was put on antibiotics, and advised to rest and modify her schedule. While at this morning's event, she became overheated and dehydrated," according to Bardack.

"I have just examined her and she is now re-hydrated and recovering nicely."

RELATED STORY Clinton collapse: Six other prominent politicians who have fallen ill on the job

Stumble

A video posted on Twitter showed Clinton appearing unsteady as she waited to get into a black van to leave the 9/11 service.

She appeared to stumble as she was helped into the vehicle, and had to be held up on either side by members of her entourage.

It was a humid day in New York, with temperatures around 82 degrees Fahrenheit (28 Celsius).

"Many of us (had) a sigh of relief when a gust of wind would come by because it was incredibly stifling," Democratic congressman Joe Crowley, who stood near Clinton for about an hour at the ceremony, told MSNBC.

Clinton walked out of her daughter Chelsea's home a few hours later, smiling and posing for pictures with a young girl before departing for her home in Chappaqua, just northeast of Manhattan.



"I'm feeling great, it's a beautiful day in New York," Clinton said.



Her schedule modification comes in the heat of a tightening race in the home stretch.



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



But any loss of valuable days on the campaign trail could juxtapose poorly against an aggressively campaigning Trump.



Democratic National Committee interim chair Donna Brazile wished Clinton "a speedy recovery," adding that "I look forward to seeing her back out on the campaign trail and continuing on the path to victory."

Health issues?

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.

The internet is awash with claims that she may have a brain tumor, Parkinson's or dementia.

Trump, 70, has said Clinton is "not strong enough to be president" and that she "lacks the mental and physical stamina" for the job.

The root of the claims lies in 2012, when Clinton was nearing the end of her State Department tenure. A stomach virus and dehydration prompted her to faint, causing what her doctor said was a concussion.

They said they found a blood clot on the brain and Clinton temporarily suffered from double vision. She later received the all-clear.

The former first lady has dismissed "conspiracy theories" about her health and pointed to a letter from her doctor declaring her fit to serve as president.

A Monday coughing spell by Clinton prompted renewed questions about whether she is physically fit for the job.

RELATED READING Barbra Streisand mocks Trump in song

'Vitality and viability'

Jennifer Lawless, a professor of government at American University in Washington, said Clinton will be under pressure to show she is not laid low by illness.

"What the Clinton campaign needs to do over the course of the next several days is demonstrate her vitality and viability. She has to be at tons of events and seem very energetic," Lawless told AFP.

Larry Sabato, a veteran political scientist at the University of Virginia, said Clinton's team should release a full health record.

"We really haven't gotten very much, essentially a letter from her doctor," he told CNN, but added that Trump should be held to the same requirement.

Despite Clinton's pneumonia diagnosis, she had a whirlwind schedule Friday that included a two-hour working meeting on national security, and a sit-down interview.

"'Powering through' illness is what women do," tweeted former Michigan governor and Clinton surrogate Jennifer Granholm.

A Washington Post-ABC News poll out Sunday shows Clinton leading Trump 46 percent to 41 percent among likely voters.