The men climbed onto the roof of the Queens Gallery, a public art gallery on the Buckingham Palace grounds that has a separate entrance from the residence itself.
The men are a part of the fathers' rights group Fathers For Justice.
The activists claimed to have breached security by creating a distraction and climbing a ladder, according to the group. One member told British television station ITV over the phone that it was "easy" and he could "have gone further."
There have been several security lapses at Buckingham Palace over the years, including one in 2004 also involving a fathers' rights protest, during which a man in a Batman costume climbed the facade of the Palace.
In 2003, Prince William's 21st birthday costume party at Windsor Castle was gate-crashed by a man dressed in a pink dress as Osama bin Laden.
In the same year a Daily Mirror reporter got a job posing as a footman.
In perhaps the most troubling incident, in 1982 Michael Fagan climbed a drainpipe and made it all the way to the queen's bedroom. She woke to find the intruder sitting on her bed. There are various reports about what happened next from security forces and Fagan himself. Many credit the queen with stalling him in conversation -- even offering him a cup of tea -- until security forces could be summoned. The Home Secretary offered his resignation over the security breach at the time.
In 2010, when Prince Charles and Camilla took a car to the Royal Variety performance, demonstrators and rioters who were staging a protest over tuition fees managed to ambush their Rolls Royce, shattering a window when the car became separated from its police escort.
An investigation into the incident took place, with security forces claiming the route had initially been clear just moments prior. Sir Paul Stephenson the Scotland Yard commissioner called the incident "hugely regrettable" and promised a "thorough investigation."
More recently in 2013 a man made it inside to be of the Buckingham Palace state rooms. The queen was at Balmoral at the time.