Brianna, from Birchwood in Warrington, was found by members of the public as she lay wounded on a path in Culcheth Linear Park at around 3.13pm on Saturday.
Police were exploring whether Brianna, who was a transgender girl, was the victim of a hate crime.
On Tuesday evening people gathered for candlelit vigils at St George’s Hall in Liverpool and at College Green in Bristol.
More vigils are due to be held across the country in coming days.
In a tribute issued after Brianna’s death on behalf of her family, they said the teenager was a “much-loved daughter, granddaughter, and baby sister”.
The statement continued: “She was a larger-than-life character who would leave a lasting impression on all that met her.
“Brianna was beautiful, witty and hilarious.
“Brianna was strong, fearless and one of a kind.
“The loss of her young life has left a massive hole in our family, and we know that the teachers and her friends who were involved in her life will feel the same.”