Boris Johnson said he wants the reportedly English jihadist who beheaded an American journalist to be killed in a bomb attack, and joined the growing calls for Britons fighting abroad to be stripped of their citizenship.
But Mr Clegg said he would listen to what the police and intelligence services said they needed to tackle the "hateful ideology" of IS rather than accepting the Mayor of London's proposals.
Speaking during a visit to India, he said: "I think the issue is incredibly important, that we should make sure that those young men ...who are attracted to this hateful ideology that draws them to the bloodshed and conflict in places like Syria and Iraq, shouldn't be able to come back and do harm on the streets of British towns, villages and cities. That is now our number one priority.
"It's what the police, the authorities and security services are working flat out on; I have huge confidence in the outstanding work they do. With the greatest respect to Boris Johnson, I will listen to what they tell me they need rather than what he says.
"We have a number of measures already on the statute book which allow us to keep a very close eye on those people who aren't in prison, aren't sentenced, but are perceived to be a threat to the UK."