US President Barack Obama urged young people to ignore cynics telling them they cannot change the world as he opened a town hall-style meeting in London.
On the second full day of his visit, Mr Obama addressed young people in Westminster and then took questions from them.
Mr Obama praised the close relationship between the US and the UK, which he said had improved dramatically since the British "burned down my house" - a reference to the torching of the White House in the war of 1812-1814.
Asked about the peace process in Northern Ireland, Mr Obama said it was an example of what can be achieved when the US and Britain work together.
Mr Obama said he was pleased that "tribal mentalities" were being broken down in Northern Ireland.
The president is also meeting Labour leader Jeremy Corbyn following days of uncertainty regarding the event.
Such a meeting between an American president and leader of the opposition would usually be routine during a trip to Britain stretching across three days, but Labour officials could only say in the past few days they "hoped" to announce a get-together was happening.
News of the meeting with Mr Corbyn, who opposes British military intervention in Syria, came as Mr Obama said the US and Britain were ready to take action to stop the Islamic State (IS) terror group securing a stronghold in Libya from which to launch attacks on Europe or America.
Mr Obama will also visit London's Globe Theatre to mark the 400th anniversary of the death of William Shakespeare on the second full day of his trip.
Speaking after talks with Prime Minister David Cameron at 10 Downing Street, Mr Obama stressed there were "no plans" to send ground troops into Libya to support the recently-established Government of National Accord (GNA).
He said both the UK and US were determined to assist the administration of Fayez Serraj with technical help, training and advice to secure borders, drive out terrorists and rebuild Libya's economy.
Mr Obama also said he and Mr Cameron would not sit back and wait while IS - also known as Isil or Daesh - built up its base in the coastal city of Sirte into a launchpad for atrocities against the West.
"There are no plans for ground troops in Libya," said Mr Obama. "I don't think it's necessary. I don't think it would be welcomed by this new government. It would send the wrong signal.
"This is a matter of 'Can Libyans come together?' What we can do is provide them with our expertise. What we can do is provide them with training. What we can do is provide them with a road-map for how they can get basic services to their citizens and build up legitimacy.
"But I do think that the one area where both David and I are heavily committed is that, as this progresses, we can't wait if Isil is starting to get a foothold there.
"So, we are working - not just with the Libyan government but a lot of our international partners - to make sure that we are getting the intelligence that we need, and in some cases taking action to prevent Isil from having another stronghold from which to launch attacks against Europe and the United States."
Libya was top of the agenda at the talks in Downing Street and is expected to dominate a summit of the G5 group in Hanover, Germany, on Monday, at which Mr Cameron and Mr Obama will discuss security threats with chancellor Angela Merkel, French president Francois Hollande and Italian PM Matteo Renzi.
Speaking alongside Mr Obama at a press conference in the Foreign Office, Mr Cameron said: "There is no doubt that the situation in Libya is immensely challenging. But we now finally have a government of national accord with whom we can work."