Baroness Golding has told a court of her alarm after two of her guests staged a Commons protest which saw flour thrown at the prime minister.
The pair were guests of Baroness Golding
The peer told the trial of Patrick Ronald Davis, 48, that she feared the purple flour might be poisonous.
Mr Davis accompanied fellow fathers' rights activist Guy Harrison, 36, who had won a VIP ticket from the baroness at a charity raffle, it was alleged.
He denies a single charge of disorderly behaviour, which Harrison has admitted.
Bow Street Magistrates' Court was told that Harrison, who was fined £600 in May for the offence, and Mr Davis were shown by the baroness to a spot in the guests' gallery, not protected by a security screen.
The peer told the court she was less than 10ft from the pair as they started their protest on 19 May.
Mr Davis unfurled a banner during the protest and blocked
the path of a doorkeeper, it is alleged.
Baroness Golding said she had been concerned by the amount of time the men had spent going to the toilet and talking to each other.
"I saw Mr Davis hold up a poster. Then Mr Harrison came down to the railings and was in the process of lifting his arm and throwing something."
Karen Jones, prosecuting, said Harrison had thrown condoms full of purple flour towards Tony Blair.
Mr Davis was alleged to have shouted Fathers 4 Justice slogans and held up a placard.
The peer added: "Theirs is a cause I support, it transpired afterwards, but I was really devastated that the Houses of Parliament could be treated in such a way by people.
"I was also concerned about what they had thrown because I could see the mist and the purple and didn't know what it was.
"I thought it might be poisonous and was worried about the people down below in the chamber and those in the gallery."
She said the security breach had been alarming.
Commons doorkeeper Ken Jones said he had been briefed shortly before the attack about the threat from terrorists and had feared a biological agent was involved.
He said he had tried to tackle Harrison, but had been blocked by Mr Davis.