Campaigners dressed as superheroes have scaled the roof of deputy Labour leader Harriet Harman's home while she and her husband were inside.
The Fathers 4 Justice activists, named as Mark Harris, 46 and Jolly Stanesby, 41, from Plymouth, climbed onto the house in south London dressed as Captain Conception and Cash Gordon.
The group, which campaigns for fathers' rights, said two more unnamed members were inside the building and had unfurled a banner from a bedroom window which read "A father is for life, not just conception".
Miss Harman and her husband, Labour party treasurer Jack Dromey, were said to be inside the house at the time.
Mr Harris said: "All we did was push open the gate which wasn't even locked, put a ladder up and climbed up.
"In this time of heightened terror alerts, I can't believe Harriet Harman has such lax security. My house is more secure than this."
The founder of the direct action group, Mark O'Connor, said: "This is the beginning of a series of protests leading up to Fathers' Day.
"We've got absolutely nowhere in terms of dialogue with the Government and if anything the situation for fathers is infinitely worse.
"Politicians don't want to tackle the elephant in the room, fatherhood, for fear of upsetting single mothers.
"They say you can abandon your children tomorrow if you pay."
The campaigners have said they will remain at the property until Miss Harman has read Mr Harris's book, Family Court Hell.
Mr O'Connor said: "They've got enough food to last them for a week."
Fathers 4 Justice have become well-known for their publicity stunts.
In 2004, two members of the group threw condoms filled with purple flour at Tony Blair, who was then Prime Minister, as he was addressing the House of Commons.
Later that year, another member of the group breached Buckingham Palace security and climbed onto a ledge wearing a Batman costume.
Fathers 4 Justice was shut down in January 2006 after hardline sympathisers were accused of plotting to kidnap Mr Blair's son Leo.
But the group was reborn four months later when campaigners invaded the set of the National Lottery draw during a live broadcast.
This latest stunt is the beginning of a series of new protests by the group, according to Mr O'Connor.
He said: "There will be an official protest in Bristol, when we'll be targeting the Child Support Agency (CSA) in a way which hasn't been seen before."
"We'll also be revisiting many of our older protests."
Last month, Miss Harman said that marriage was irrelevant to government policy.
Her comments led to accusations that Labour was trying to destroy traditional family life in Britain, and in the process condemning children to lives of poverty in single-parent households.
A Scotland Yard spokeswoman said: "Officers are currently in attendance at the location and are speaking to the men."