The mayor of London has hit out at US presidential candidate Mitt Romney for comments suggesting Britain is not ready to stage the Olympic Games.
Boris Johnson fired up a large crowd in Hyde Park, saying: "There's a guy called Mitt Romney who wants to know whether we're ready. Are we ready?"
Earlier, Mr Romney backtracked after meeting UK PM David Cameron and predicted a "very successful" Olympics.
The Republican will take on Barack Obama in November's election.
Mr Romney's remarks came on the first official day of a week-long tour that is to include stops in Israel and Poland.
It is the first foreign visit Mr Romney is making as a presidential candidate and correspondents say his campaign is aiming to boost the foreign policy profile of the former governor of Massachusetts.
In the contest for the Republican nomination, Mr Romney made much of the fact that he delivered a successful Winter Olympics in Salt Lake City in 2002, when he was chief executive of the event.
At the end of a day that saw the Olympic torch make its way one last time through the streets of London, Mr Romney's remarks brought a caustic comment from Mayor Boris Johnson in front of a huge sun-baked crowd in Hyde Park.
"I hear there's a guy called Mitt Romney who wants to know whether we're ready," Mr Johnson told a crowd of tens of thousands of people who had gathered in Hyde Park, in central London.
"He wants to know whether we're ready. Are we ready? Are we ready? Yes, we are!"
Earlier, in an interview with US network NBC, Mr Romney said London's difficulties with security guards and threats of border staff strikes were "obviously... not something which is encouraging".
"It's hard to know just how well it will turn out," Mr Romney said, referring to the Olympic Games that kick off in London on Friday.
That prompted a response from the prime minister, who praised London's readiness and said it would be easier to organise an Olympic Games "in the middle of nowhere" - a remark widely seen as a reference to Mr Romney's stewardship of the 2002 Salt Lake City Winter Games.
But after his talks with Mr Cameron, Mr Romney said mistakes were to be expected and he was sure the London Games would be a success.
Speaking to reporters outside 10 Downing Street, Mr Romney said he "applauded the work of the organising committee in bringing the Olympic experience right into the heart of London".
"My experience as an Olympic organiser is that there are always a few very small things that end up going not quite right in the first day or so," Mr Romney said, adding that those get "ironed out" and "overwhelmed" by the accomplishments of the athletes.
Back in the US, supporters of Mr Romney insisted the day's events were not relevant to the presidential race at home.
"We're not worried about overseas headlines, we're worried about voters here at home in America," said Louisiana Governor Bobby Jindal, a leading Romney ally.
Yet the back-and-forth attracted attention on Twitter, and did dominate US TV coverage of Mr Romney's day in London.
Senate majority leader Harry Reid - a Democratic opponent of Mr Romney - delivered a blunt verdict.
"It's not good for us as a country - it's not good for him - but as a country to have somebody that's nominated by one of the principal parties to go over and insult everybody," Mr Reid told the Huffington Post .
Mr Romney also held meetings with former prime minister and Middle East peace envoy Tony Blair, Labour leader Ed Miliband and other senior figures.
Mr Romney, who at one point addressed Mr Miliband as "Mr Leader", affirmed the commitment of both countries to peace, and a "stronger and growing economy".
A spokesman for Mr Blair said the two men discussed the Middle East peace process, Iran and the situation in Syria.
Mr Romney also said he met the head of Britain's foreign intelligence agency MI6 - an unusual disclosure - when asked about his discussions on Syria.
While in London, Mr Romney also attended fundraisers that correspondents say could raise about $2m (£1.3m) for his campaign war chest.
On Friday, Mr Romney is to attend the opening ceremonies of the Olympic Games before departing for Israel on Saturday.