"The threat that Donald Trump poses is so dramatic to our country, to our democracy and our economy," the former secretary of state told CNN in a live interview.
Asked directly whether she believes the celebrity billionaire and presumptive Republican nominee is qualified to be president, she said: "No, I do not."
Clinton pointed to Trump's recent comments on being willing to meet with North Korea 's leader, whom she labeled a "despotic dictator," suggesting the United States pull out of NATO , or allowing certain allies to have nuclear weapons.
Such positions are "dangerous," she said, adding that the volume of Trump's provocative and alarming statements since he launched his presidential campaign , including his call for a ban on all Muslims entering the United States, suggest he is not making impromptu remarks.
"I think if you go through many of his irresponsible, reckless, dangerous comments, it's not just somebody saying something off the cuff," Clinton said.
The harsh criticism came as Clinton all but declared victory in her nomination battle with Bernie Sanders .
"I will be the nominee for my party. That is already done," she said. "In fact there's no way that I won't be."
Sanders, a US senator from Vermont, insists he is still locked in a fierce battle for the nomination and has vowed to take the race all the way to June 7, when California, the largest state in the union, holds its primary.
But Clinton essentially said it was time for Sanders to accept the inevitability of her winning the nomination.
"I'm three million votes ahead of him and I have an insurmountable lead in pledged delegates," she said.
It was perhaps Clinton's most forceful declaration to date that the race is basically over, as she turns her focus on Trump and their general election showdown.
She also expressed confidence that Sanders will unify behind her, "just as I did with senator (Barack) Obama" during their intense battle for the nomination in 2008, once the Democratic race comes to a halt.
"He has to do his part to unify," she said of Sanders.