They are demanding more domestic violence services
Eight activists entered the building via an open window on Saturday afternoon, as 150 rallied outside.
Sisters Uncut have called themselves the "voice of domestic violence survivors", and are occupying the space as part of ongoing action to demand more domestic violence services from the government.
"We wanted to reclaim this former prison because it was a site of extreme state violence," said Sisters Uncut member Naisha Garcha, 25. "And the fact that the prison has been closed for a nearly a year and the government seem to be intent on building luxury flats on the site instead of anything for the community."
"We've been planning this basically since the prison closed. You'd think the police would have better things to do, especially this week."
The building has been surrounded by dozens of police officers who are treating the area as a crime scene, but no arrests have been made.
At one stage a pizza delivery man was turned away from the premises by police who said it was illegal for him to deliver food to the occupiers.
Other Sisters Uncut members managed to winch food and water up to the activists who were on the roof of the building, and were advised that their actions were within the law.
They plan to maintain their "reclamation" for a week, holding workshops on women’s well being, self-defence and legal rights.
"I think everyone inside is feeling okay," said 20-year-old Sisters Uncut member Melissa Sur. "I think they're really happy that they got in.
"We want this to be a women's centre. The idea is to hold a seven day festival – to do the opposite of what a prison does and bring people together, help families, help women."
Holloway Prison, once the largest women's prison in Western Europe, was closed suddenly in 2016.
It was infamous for its mistreatment of prisoners, including suffragettes in the early 20th century. In January last year Sarah Reed, a black woman who had previously been the victim of police brutality, was found dead in her cell under circumstances campaigners have deemed suspicious.
The prison was closed for good in July 2016.
According to charity Women in Prisons, 46 per cent of women in prison report having suffered domestic violence and 80 per cent are incarcerated for non-violent crimes.