Frustrated by the lack of progress on voting rights, in 1907, she became an active member of the more radical WSPU.[4] On 18 November 1910, Anderson joined her mother, Emmeline Pankhurst, Alfred Caldecott, Hertha Ayrton, Mrs Elmy, Hilda Brackenbury, Princess Sophia Duleep Singh, and 300 women to petition Prime Minister Asquith for voting rights.[5] The protest became known as Black Friday, due to the violence and sexual assault the protesters faced from the police and male bystanders.[6] More than one hundred women were arrested, including Anderson, but all were released without charge.[6] In 1912, she was imprisoned in Holloway, briefly, for her suffragette activities which included breaking a window by throwing a brick.