Protesters from campaign group Fathers 4 Justice entered the London offices of a children's charity on Monday.
Palace protester Jason Hatch was involved in Monday's demonstration
Demonstrators blasted horns and shouted slogans outside the National Society for the Prevention of Cruelty to Children, while four people entered.
A 71-year-old grandmother was inside the Shoreditch offices for an hour.
Protesters alleged the NSPCC ignores the plight of 100 children a day who lose contact with their fathers in the courts, a claim the charity has denied.
An NSPCC spokesman said: "The NSPCC is working with organisations which support fathers in residence and contact disputes.
"Over the last year, the NSPCC National Child Protection Helpline has helped more than 1,500 men calling about family relationships."
The NSPCC said its research showed that mothers were responsible for 49% of violent incidents against children with fathers responsible for 40%.
During Monday's protest, a smoke flare was let off in the lobby and two women climbed on top of the revolving door leading inside during the peaceful protest.
Later a line of police stood outside and managed to persuade the campaigners to move across the road.
Fathers 4 Justice founder Matt O'Connor said: "Depriving children of a father is an abuse of that child's right to family life and constitutes 'emotional cruelty' - a term used by the NSPCC to describe child abuse.
"We also strongly criticise the continuing portrayal of men as violent
abusers when the vast majority of abuse perpetrated on children is by their
mothers, according to the NSPCC's own research."
Grandmother Dympna Hill, 71, from Brentwood, Essex, was inside for nearly an hour.
She said: "My main protest is about cases where the father should have access to the child, according to the court, but the mother will not let him. That happens a lot."
Police Inspector Liam Harrington said there had been no arrests and no-one was hurt.
Mr Harrington said Jason Hatch who scaled the balcony at Buckingham Palace dressed as a superhero in September had been one of the demonstrators.