The 2016 American election has at times been "downright embarrassing", US Secretary of State John Kerry has said.
With just over a week to go before people vote for Hillary Clinton or Donald Trump, Mr Kerry said the election had stepped "out of any norm" but added he believed his country would "turn the corner".
Before they were invited to ask questions, sixth-formers from across London had been warned there were "strict rules" around what Mr Kerry could talk about in relation to the election at the event alongside mayor Sadiq Khan at City Hall.
Asked about how the election has impacted on US relations abroad, Mr Kerry said: "This election has been difficult for our country's perception abroad. There are moments when it is downright embarrassing."
He added that there are "times when it steps out of any norm" he has known.
"I could never imagine debates that were not focused on real issues," he said.
His comments come as Mrs Clinton has had to again go on the defence over her use of a private email system, saying she will not be "knocked off course" in the election's final days.
The controversy over Mrs Clinton's email practices while she served as secretary of state has dogged her for more than a year.
Meanwhile, Republican candidate Mr Trump has been condemned for comments he made about women in a recording from 2005.
Mr Kerry said: "The way it's made it difficult for me is that when you sit down with some foreign minister in another country, or with the president or prime minister of another country, and you say, 'Hey, we really want you to move more authoritatively towards democracy', they look at you ... they're polite, but you can see the question in their head and in their eyes. And in their expressions. It's hard.
"Or you run in and you say, 'By the way, it's really important you guys get your budget passed', and I can see the quizzical look at us.
"The last time we passed a budget was I don't know how many years ago. We do a continuing resolution nowadays. We don't do the normal process.
"So this is a difficult moment. But the one thing I would say to you is the great thing about the United States is that it has amazing resiliency. It has an incredible ability to absorb something like this and it will come out and in my judgment it'll come out stronger.
"We'll focus and we'll know where we're going and I'm really confident about the longer term future. But sometimes we go through these really rough moments politically and you just have to fight through them."
Mr Kerry added: "I'm not down in my cups about it. I think we'll turn the corner as long as we're offering decent alternatives and as long as you are embracing the system and fighting to change it and make it better."
Earlier this year, Mr Khan said Mr Trump should come to London to meet moderate Muslims who would challenge his "ignorant" views on Islam.
The mayor, who is a Muslim, was involved in a public row with Mr Trump who called the Labour politician "rude" and "nasty".
Mr Khan's clash with the White House hopeful followed Mr Trump's call for Muslims to be banned from the US - although the presidential candidate said he would allow the London mayor to visit the States.
After the row, Mr Trump's bid was plunged into crisis when a tape emerged featuring Mr Trump talking about groping women and saying he could "grab them by the pussy" because he was a celebrity.
Asked if he would still welcome Mr Trump if he won the election, Mr Khan said: "Well, let's wait and see what happens in the presidential election. I'd welcome any US president to my great city of London. And I look forward to Hillary coming here as soon as she's the president."