The two-child cap for tax credits and the so-called rape clause constitute an attack on women, campaigners in Belfast said.
Protesters held a city centre rally outside the offices of the Communities Department responsible for implementing the policy in Northern Ireland.
The UK Government policy limits families to claiming tax credits for their first two children.
One of the exemptions is the so-called ''rape clause'', which requires women to prove their third child was conceived through rape or during an abusive relationship to qualify for the benefit.
Campaigner Louise Kennedy said: "Requiring women to prove that they conceived a child through rape to access child tax credits is cruel and inhumane.
"The policy will re-traumatise many victims of rape, while others will simply not come forward to claim what they are entitled to, leaving them and their children in poverty."
In announcing the change the UK Government said it wanted to limit child tax credit to the first two children because it wanted "people on benefits to make the same choices as those supporting themselves solely through work".
It introduced an exemption for a child born through "non-consensual" conception.
Women's Aid Federation policy worker Ms Kennedy said: "As this policy punishes some of our society's most vulnerable, it's our duty to stand together and support them. We have to give victims of rape and domestic abuse a voice."
Kellie Turtle, a women's sector lobbyist, said they were not prepared to accept welfare cuts aimed at low income families, claiming they demonised women and stigmatised children.
"The Department for Communities is about to implement a measure that will have a terrible impact on women and children.
"While we are fighting this policy at Westminster, in the absence of a functioning Executive at Stormont we have no choice but to take to the streets and make our voices heard."
Organisations and individuals continue to lobby MPs to call for the two-child cap to be repealed.
Northern Ireland MPs signed an Early Day Motion at Westminster opposing the rape clause during the previous mandate.