FiLiA CEO Lisa-Marie Taylor added that the charity “campaigns for a world free from patriarchy where all women and girls are liberated.”
“The activities of a small, violent minority will not diminish the spirit of sisterhood and solidarity which FiLiA embodies,” she said.
More than 2,400 people are expected to attend the conference to hear from 250 speakers from around the world, according to organisers.
Talks will cover a “broad spectrum” of topics, including “anti-racism, women’s health, male violence, political organising, lesbian lives, migration and class”, they added.
Sussex Police urged anyone who witnessed what happened or has relevant information to get in contact.