Senator Kamala Harris on Saturday (local time) apologised following criticism that spurred after the California Democrat laughed and responded saying "Well said" to a lengthy question from a voter who had called President Donald Trump's actions "mentally retarded".

Following this, Harris' presidential campaign account tweeted an apology, calling a video of the incident that was circulated on social media "upsetting", and denied she had heard the offensive term, CNN reported.

"I didn't hear the words the man used at that moment, but if I had I would've stopped and corrected him. I'm sorry. That word and others like it aren't acceptable. Ever," she wrote on Twitter.

The questioner at the Londonderry town hall in New Hampshire on Friday identified himself as hailing from the south Indian city of Chennai, which is also Harris' mother's hometown, and said, "Somehow a racist bigot gets into the White House and then he says if you're not my colour you need to go back to your own country."

"So I am scared for this country. I am scared for the people of colour in this country," the man was heard as saying in the video shared on social media.

He then asked Harris what she would do "in the next one year, to diminish the mentally retarded action of this guy?"

The man received a round of applause and laughter from the crowd. Harris too laughed and responded, "Well said, well said."

"I plan to win this election, I'll tell you that," Harris said to applause.

"It's going to be about working as hard as I possibly can to get there, because over the course of this next year that's what it's going to take to unseat him," she told him.

"Thank you for having the courage to stand up and say it is that there are a lot of people living with extreme fear right now in our country, extreme fear," the senator added.

The controversy garnered after disabled health care advocate Kendally Brown tweeted out a 29-second clip of the conversation between Harris and the man, and wrote, "I hate amplifying content I know the right will seize on and twist for their own hypocritical gain, but this hurts my heart. #CripTheVote."

Before the California Democrat issued the apology, she spoke with various media organisations and called the term offensive.

Harris denied she had heard the term and said, "You would think that in the year 2019, people would have a much better understanding how hurtful a term like that can be -- but also the history behind it, which is the history of really ignoring the needs and the realities and the capacity of our disabilities community.