Mitt Romney has been rebuked by the US Olympic legend Carl Lewis after the presumptive Republican presidential nominee questioned whether London was fully prepared for the Olympic Games.

As Romney was mocked on US television and by the London mayor, Boris Johnson, in front of a crowd of 60,000 people, Lewis called on him to return home.

"Seriously, some Americans just shouldn't leave the country," the nine-times Olympic gold medal winner told the Independent.

Asked whether London was ready to stage the Games, Lewis said: "Of course London is ready."

Romney stumbled into a diplomatic disaster on the first stage of his first official overseas visit as the presumptive Republican nominee when he told NBC there were "disconcerting" signs that London was ill-prepared for the Olympics.

An irritated David Cameron slapped Romney down by saying that Britain was staging the Games in a "bustling" city and not "in the middle of nowhere". This was intended as a light-hearted reference to the Salt Lake Winter Olympic Games, which were famously rescued by Romney in 2002. Some in Whitehall said his performance on the diplomatic stage was more woeful than Sarah Palin's four years ago.

Romney tried frantically to row back on Thursday as he said the Games would be a great success. But the damage had been done.

Romney was teased remorselessly on US television. The Rachel Maddow Show on MSNBC ran the story as #romneyshambles – a reference to the phrase omnishambles from political comedy The Thick of It. The stand-in presenter opened the show saying: "Let's just be honest here. Today was a terrible, horrible, no good, very bad day to be Mitt Romney. Even if you consider yourself the staunchest Mitt Romney supporter out there, you probably have to admit that today did not go exactly according to plan. The kick-off of Mitt Romney's big overseas trip has kind of been a disaster."

On Friday morning, the UK culture secretary, Jeremy Hunt, referred to Romney's gaffes when he was asked about London's readiness for the Games. Hunt told the ITV show Daybreak: "The person I care about more is Jacques Rogge, the president of the International Olympic Committee, who said London was the best-prepared city he had ever seen.

"When we have the opening ceremony tonight and we tell the world that eight of the world's top 10 sports were either invented or codified in Britain – and only two in America – I hope Mr Romney is watching."

And Boris Johnson, the London mayor, joined in when he mocked Romney at a rally in Hyde Park on Thursday evening. "There are some people who are coming from around the world who don't yet know about all the preparations we've done to get London ready over the last seven years. I hear there is a guy called Mitt Romney who wants to know whether we are ready."

Johnson then asked whether London was ready. As the crowd cheered, the mayor declared: "Yes we are. The venues are ready, the stadium is ready, the aquatic centre is ready, the velodrome is ready, the security is ready, the police are ready, the transport is ready and our Team GB athletes are ready.

"There will be more gold, silver, bronze medals than you need to bail out Greece and Spain together."

As Johnson piled the pressure on Romney, the Republican was at the nearby Mandarin Oriental Hyde Park for a fundraiser in central London on Thursday night. The dinner was estimated to have raised at least $2m for his campaign, according to the New York Times.