Conservative leader David Cameron said it was too soon to tell whether Mr Blair had misled the country about the reasons for going to war.
"Clearly, some of the information that was put in front of Parliament, the dodgy dossier for instance was just unacceptable and was wrong and shouldn't happen again but I think we have to wait for Chilcot's full report before we come to a full conclusion."
Lib Dem leader Nick Clegg, whose party opposed the war, said he "was not sure if people have learned that much which is substantially new from Tony Blair", but said it was a "good thing" he had appeared.
"Because that makes it all the more difficult for Britain ever to be taken to war again, simply on the personal whim of the prime minister," he added.
SNP Westminster Leader Angus Robertson said: "No matter how skilfully he ducked and dived today, Tony Blair's legacy will forever be that of the illegal, immoral Iraq war.
"Chillingly, the ex-prime minister showed no regret for his dubious decisions which led us into the worst foreign policy disaster in modern times."