Tony Blair became "very steely, very focused" immediately after the September 11 attacks, his former chief of staff said on Saturday.
In an interview marking 10 years since 9/11, Jonathan Powell told BBC Breakfast: "He realised that things had changed. Many people took longer to get there.
"A lot of people didn't realise quite how much this changed attitudes in America. For them, it was another Pearl Harbour. It changed everything."
Blair was in Brighton on the day preparing for a speech for a TUC conference. He was alone in a room making final tweaks when an aid interrupted him and told him there was something he must watch on TV.
After the second plane crashed into the twin towers, he knew it was a terrorist attack.
"He really became very steely, very focused. I think he was the first on television to express our sympathy as a country, but also the world's sympathy with the United States and to stand alongside them," said Powell.
"Many Americans still remember what he said at the time and that's why he's still quite popular in the US because he stood out there and said what the world thought about it."
Meanwhile, Powell said that Downing Street was unable to contact President Bush until the day after the terrorists struck.
Bush was "a very shaken man", he said. "He wasn't sure what to do. His voice was quavering. He was in a state almost of shock."
In a separate interview with the Radio 4 Today programme on Saturday, Blair insisted "significant blows" had been struck in the war on terror but admitted, "it's not over".
Speaking from his London home on the eve of 9/11, he defended the actions he took in the wake of the atrocity and said it was "deeply naive" to believe the response of the west had radicalised extremist Muslim factions.
He said radicals "believe in what they believe in because they believe their religion compels them to believe in it".
He told the terrorism threat would only end when "we defeat the ideology".
"I think it will take a generation, but the way to defeat this ideology ultimately is by a better idea, and we have it, which is a way of life based on openness, democracy, freedom and the rule of law."
