A protester climbed onto the roof of the Houses of Parliament on Tuesday and unfurled a banner.
It wasn't immediately clear what the activist had written on the banner, but the group Fathers 4 Justice said one of their members were carrying out the protest. The group campaigns for greater rights of access to children by divorced fathers.
Sky News TV showed pictures of the activist atop Westminster Palace, with several police officers standing nearby, on a lower part of the roof.
A spokeswoman for London's Metropolitan Police had no immediate comment on the incident, the AP says.
In May of last year, a father's rights activist threw a condom filled with purple flour at Prime Minister Tony Blair as he spoke in the House of Commons, Parliament's lower chamber. The stunt prompted a major review of parliamentary security.