The climate change protester who "slimed" Lord Mandelson with green custard has been arrested, police said today.
Scotland Yard said Leila Deen, 29, attended a central London police station "by arrangement" yesterday. She was released on bail until early April.
The anti Heathrow expansion campaigner accosted the Business Secretary outside the Royal Society in London on Friday morning.
She flung a cupful of custard in his face, before walking away unchallenged and talking to journalists.
Lord Mandelson today expressed "surprise" that Ms Deen had been able to bypass security, and added that he had suffered a rash after the incident.
In his previous role as Northern Ireland Secretary, Lord Mandelson received round-the-clock police protection, but he later dropped it. The dangers of the job were highlighted last night with the shooting dead of two soldiers in Antrim.
Asked on BBC 1's Andrew Marr show today whether he was worried at the security breach, Lord Mandelson said: "It could have been something nastier. It did actually leave a slight sort of irritation on my face.
"I was slightly surprised that she could just saunter off without being apprehended. I was also slightly disappointed that some in the media just took her into their studios and started interviewing her."
He went on: "It's a bit surprising, but we do not live in a police state in Britain and thank goodness for it."
The Yard initially said it was not going to investigate because no complaint had been made against Ms Deen, a member of the Plane Stupid campaigning group.
However, an inquiry was later launched after former Deputy Prime Minister John Prescott suggested the incident amounted to assault.
Mr Prescott - who punched a protester after receiving an egg in the face during the 2001 general election campaign - welcomed the arrest.
"I'm glad to see that the police are now taking the attack on Peter Mandelson seriously and that an arrest has been made," he said on his Go Fourth blog.
"I spoke out publicly on Friday as I felt that the media seemed to be presenting it as an amusing incident and the police said they wouldn't act until they had a 'formal complaint'.
"I hope now that if something like this happens again, police forces across the country will investigate immediately."
Speaking before news of the arrest became public, Home Secretary Jacqui Smith told Sky News' Sunday Live: "I don't think in a democracy where people are able to speak up that anybody should chuck custard at anybody in the street.
"Peter himself made the point that the protester was so busy chucking the custard she wasn't actually explaining what the point of her protest was."
But she insisted it was important people did not overreact to the incident, and said security arrangements offered to ministers were "appropriate".