A fathers' rights protester has been fined £600 for throwing flour at Tony Blair during last week's prime minister's questions.
Mr Harrison and Mr Davis appeared at a news conference on Thursday
Mr Blair was hit by a condom filled with purple flour while speaking in the Commons, prompting a security review.
Guy Harrison, 36, of Ashurst, Sussex, admitted a public order offence at London's Bow Street Magistrates' Court.
A second man, Patrick Ronald Davis, 48, pleaded not guilty to the same offence and will return to court next month.
The chamber was evacuated for more than an hour after the flour was thrown.
Campaign group Fathers 4 Justice claimed responsibility.
The incident happened despite a £600,000 bullet-proof security screen being installed in the chamber at Easter.
Other new security measures costing £5m are also being introduced, including a new entrance to the Palace of Westminster.
Before this week's prime minister's questions Speaker Michael Martin told MPs he had suspended, until further notice, the right for MPs to bring guests into galleries not covered by the security
Outside the court on Wednesday Mr Harrison said he had not seen his child for three years.
"All Mr Blair needs to do to stop us being here is let us have equal rights to our children," he said.
A cross-party group of MPs have condemned the media coverage of last week's flour bomb attack.
In a Commons motion they said coverage given to Fathers 4 Justice, which claimed responsibility for the attack, "rewarded" them for their actions.
The motion was tabled by Labour's Ann Cryer, Tories Bob Spink and David Amess and Lib Dem Andrew George.