Campaign staged on Palace balcony
A Fathers 4 Justice campaigner dressed as Batman is staging a protest on the Buckingham Palace balcony.
Jason Hatch, 33, entered the grounds and scaled a wall at 1500 BST, said group spokesman Matt O'Connor.
A stand-off developed as police cordoned off the area and tried to coax the protester down.
The security breach comes after a newspaper reporter got a job as a servant before the visit of President George Bush last November.
Mr O'Connor said: "We've got a guy dressed as Batman who's on Buckingham Palace on a balcony. He legged it past the armed guards."
The protester, from Gloucester, stood on a ledge to the right hand side of the main balcony and unfurled a banner which read "Super dads of fathers 4 justice".
He is believed to have two children, aged four and five, who he has not seen for several years.
Also on the banner are the words: "Fighting for your right to your kids".
A group of Fathers 4 Justice campaigners are believed to have distracted police by climbing the front gate, while Mr Hatch climbed over the outer fence.
Fellow protester Dave Pyke, 48, dressed as Batman's sidekick Robin but was stopped by armed police as he tried to scale the fence.
He told BBC News Online: "We are totally untrained, just ordinary guys and if we can get in there, anybody can get in there."
Mr O'Connor said the incident was to highlight Monday's court appearance of campaigners accused of staging a flour-throwing protest in the House of Commons.
"This is planned to coincide with the trial of Ron Davies and the fun powder plot. I've literally come from the trial at Bow Street Magistrates today."
Fathers 4 Justice was set up by fathers who have either no access, or very limited access to their children, and feel they have been treated unfairly by courts.
BBC News Online's Finlo Rohrer, at the scene, said there were about 200 people watching, including some protesters, behind a police cordon about 30 yards from the Palace gates.
A Buckingham Palace spokeswoman said: "There's a man there, but it's a police matter. I'm not sure where he is but I can hear him shouting."
The Queen was not at the Palace as she is at Balmoral in Scotland, the spokeswoman added.
The Daily Mirror's former royal correspondent James Whitaker said palace staff were "gobsmacked" and Home Secretary David Blunkett had to take responsibility.
He told BBC News 24: "It's absolutely incredible that someone's managed to do this. This is the balcony where on the great days of state and when someone gets married in the family, they stand on this balcony and wave to everyone.
"It's a bit of a holy of holies there and if the Queen has heard about this, she would be absolutely livid."
John O'Connor, a former Scotland Yard Flying Squad commander, said this breach was an "absolute disgrace".
"They are more concerned about the changing of the guard and all the ceremonies than taking security seriously. No-one there seems to have the ability to stop intruders," he told BBC News 24.
Royal security was reviewed after Daily Mirror reporter Ryan Parry got a job at Buckingham Palace as a servant before President Bush stayed there last November.
A report was published a year ago into the breach of security during Prince William's 21st birthday party at Windsor Castle.
Comedian Aaron Barschak set off six alarms and appeared on CCTV several times without sparking a response when he gatecrashed the party at the castle, it found.