Mr Livingstone is always outspoken
The Conservatives and Liberal Democrats round on the London mayor for his verbal assault on US President George W Bush.
Mr Livingstone called the US leader "corrupt" during an address to schoolchildren in a debate on the Iraq war.
He said he would get as much pleasure from Mr Bush being forced out of office as he had done from the downfall of former Iraq leader Saddam Hussein.
But opposition politicians said the remarks would be an obstacle in encouraging US tourists to return to London, just days before the mayor launches a tourist drive for the capital.
Conservative mayoral candidate Steve Norris described Mr Livingstone's attack on President Bush as "utterly irresponsible".
"The red mist comes down and his judgement flies out of the window. He has no right whatever to insult President Bush," he said.
"He has every right to his own view but not to express it when he is mayor of this city."
I've never heard of the fellow
White House spokesman Ari Fleischer on Ken Livingstone
Mr Livingstone was answering questions on the Iraq war and other subjects during a two-hour meeting with 200 schoolchildren at City Hall on Thursday morning.
After making a pointed reference to Mr Bush, he was asked by Channel 4 broadcaster Krishnan Guru-Murthy, who was chairing the meeting, to explain his making a personal attack on the US president when he disliked answering personal questions himself.
He said Mr Bush was a coward who had avoided serving in the Vietnam war only through the influence of his father.
"You have got this super-patriotic hawk who was a coward when his country was actually involved in a war which he politically supported."
The mayor said: "I think George Bush is the most corrupt American president since Harding in the Twenties.
"He is not the legitimate president."
Comments played down
Liberal Democrat mayoral challenger Simon Hughes MP said the mayor's comments were unacceptable, particularly from a politician who decried attacks when they were launched against him.
"Just one week before London launches a campaign to bring more tourists to
our city, particularly Americans, London's Mayor has undermined these efforts to make them welcome.
"Mayors of London should promote the capital and attract people to it, not make enemies and not insult the leaders of the very countries whose people we are trying to attract."
But Mr Livingstone later played down notions his comments could have an effect on tourism.
The mayor said it was as ridiculous as thinking British tourists would be put off some holiday destinations because of the opinions held of Tony Blair by local politicians.
Asked about Mr Livingstone's comments, White House Press Secretary, Ari Fleischer, said: "First of all, I've never heard of the fellow. Second, I'm not going to dignify it with a response."
And in a statement, officials from the US embassy in London, said: "Mayor Livingstone's opinions about the United States are a matter of complete indifference to the American embassy, the American government and the American people."