Ms Harris made the remarks at the funeral for 86-year-old Ruth Whitfield, the oldest of the 10 people killed in a racist attack at a Buffalo supermarket.
Ms Harris, addressing mourners, said this was the moment for “all good people” to stand up to injustice.
“I do believe that our nation right now is experiencing an epidemic of hate,” Ms Harris said, adding “we will not let those people who are motivated by hate separate us or make us feel fear.”
The vice president then visited a memorial for the victims outside the supermarket where president Joe Biden and first lady Jill Biden had placed flowers on 17 May.
She told reporters that the Biden administration is not “sitting around waiting to figure out what the solution looks like” to the gun violence problem in the country.
“We know what works on that. Let’s have an assault weapons ban,” she said, reiterating the need for background checks and a ban on assault weapons.
“An assault weapon is a weapon of war with no place, no place in civil society. Background checks: Why should anyone be able to buy a weapon that can kill other human beings without at least knowing: Hey, that person committed a violent crime before, are they a threat against themselves or others?” Ms Harris said.
She urged the nation to come together and “stand strong,” identifying “diversity as our unity.”
Her impromptu speech comes in the wake of two mass shootings in the US.
On 14 May, 18-year-old Payton Gendron, who is white, shot at 13 people at the Tops Friendly Market in a predominantly black neighbourhood. Only three people survived.
He is charged with first-degree murder and is being held without bail. Federal authorities are reportedly investigating the mass shooting as a hate crime.
Last week, another gunman burst into Robb Elementary School in Uvalde, Texas and killed 19 children and two teachers.
“This is a moment that requires all good people, all God-loving people to stand up and say we will not stand for this. Enough is enough,” Ms Harris said during the service.