Heather Heyer's mother says the white supremacists and hate groups responsible for her daughter's death didn't shut her up ... they made her message that much louder. 

Susan Bro gave a powerful speech Wednesday at her daughter's memorial service in Charlottesville's Paramount Theater, and served up a rallying cry for what her daughter died for ... standing up against hate.

Susan told a packed room ... "They tried to kill my child to shut her up. Well guess what? You just magnified her." She added ... if people continue Heather's legacy of fighting racism and hate, it'd make her daughter's death worthwhile. 

Susan ended with a powerful message ... "I'd rather have my child. But, by golly, if I gotta give her up, we're gonna make it count." 

20-year-old James Alex Fields Jr. -- an alleged neo-Nazi sympathizer -- has been charged with murder in connection to the crime.

Susan did not address Donald Trump's controversial remarks regarding blame in Charlottesville on "both sides" ... but the Prez did mention Heather today.