"Politicians get a lot of death threats and they have a need to feel safe and I compromised that in that moment by touching him (Sir Keir)," he said.
Mr Ashmawi told host Ali Milani - a former Labour party candidate who tried to to unseat Boris Johnson at the 2019 election - that he was "sorry" for making physical contact with the leader of the opposition.
He said: "I think it's absolutely fine to pour glitter on someone and to go on to the stage.
"I just think it's the physical contact that crossed the line there."
In a statement, Mr Ashmawi added: "The glitter was, of course, a lovely, peaceful spectacle and I'm still finding it everywhere."
Sir Keir's speech on Tuesday was interrupted by a protestor wearing a T-shirt linking him to a group called People Demand Democracy.
The party leader held the activist away from the microphone with his right arm before security arrived.
Mr Ashmawi continued to shout demands for changes to the UK's parliamentary democracy.
Sir Keir later called the protestor an "idiot" for trying to interrupt his speech but added he feared "it could have been a lot worse".
Labour shadow justice minister Shabana Mahmood has said she expects "questions are being asked" surrounding how security allowed the protestor to get on stage.
"We will want to make sure that nothing like that can ever happen again," she told OceanNewsUK PM programme.
