UPDATE: Nick Clegg has issue a statement clarifying his comments on LBC. He said: "I completely condemn all forms of domestic violence. As I said on the radio, my instinct would always be to try and protect the weaker person, to try and protect the person who otherwise would be hurt."
Over the weekend pictures of Saatchi emerged which showed him with his hands around his wife Lawson's neck.
The deputy prime minister was asked by a caller to his LBC Radio show on Thursday morning whether he would have stepped in had he been in the restaurant.
"What a difficult question. I find it so difficult to imagine," Clegg said. "I don't know what happened, I'm like you, I don't know what happened."
"When you see a couple having an argument, most people just assume that the couple will resolve it themselves. If, of course, something descends into outright violence that is something different.
"I just don't know, there was this one photograph, I don't know whether that was a fleeting thing. I'm really sorry Elizabeth, I am at a loss to put myself into that position without knowing exactly [what happened].
"You are asking me to comment on photographs everybody has seen in the papers - we don't know if that was a fleeting moment so I'd rather not comment on a set of events that I wasn't."
Asked about Clegg's comments, the prime minister's official spokesperson said he could not comment on the specific incident, but added: "Domestic violence should be condemned in the strongest terms. Full stop."
Yvette Cooper, Labour's shadow women and equalities minister, said: "Clegg shows how little he understands violence against women - too often dismissed as fleeting or unimportant when it is a hidden crime. Saatchi accepted caution for assault. Ministers should condemn all violence against women and show they take it seriously. Clegg failed today."
The Lib Dem leader's comments also provoked an angry backlash from other MPs, with his remarks condemned as "disgraceful" by one MP and his stance on domestic violence criticised by another.
On Tuesday Saatchi said he had accepted a police caution as he did not want the the altercation "hanging over" them.
The 70-year-old art collector has dismissed the incident as "a playful tiff" but admitted the pictures, in which his wife appears to be grimacing as he holds her outside Scott's in Mayfair, looked "horrific".