- Video report by ITV News Correspondent Juliet Bremner
Donald Trump's apparent willingness to side with Russian President Vladimir Putin over his own intelligence agencies has been met with disbelief in many circles.
US intelligence agencies believe Russia tried to assist Mr Trump's 2016 White House bid using cyber attacks and planting fake news stories on social media, and last week 12 Russian intelligence officers were indicted by the Mueller investigation on hacking charges.
But at a press conference after the two leaders met in Helsinki on Monday, Mr Trump said he didn't "see any reason why Russia would interfere in the 2016 election" and that, although he has "great confidence" in US intelligence, Mr Putin had been "extremely strong and powerful in his denial".
The US president also repeatedly denounced the special counsel investigation into Russian election meddling, saying the probe was a "disaster" for the country and had negatively affected relations with Moscow.
Back in Washington, D.C., Mr Trump's remarks, and his warmness towards the Russian leader, have been met with incredulity by both Republicans and Democrats.
Republican House Speaker Paul Ryan said there was "no question" Russia interfered in the 2016 presidential election.
"The president must appreciate that Russia is not our ally," Mr Ryan said, adding that Russia "remains hostile to our most basic values and ideals".
Senator John McCain, who ran against Barack Obama in the 2008 presidential campaign, described the press conference as "one of the most disgraceful performances by an American president in memory".
Another Senior Republican, Senator Lindsey Graham said meddling and collusion were not the same thing and that Russia would see Mr Trump's approach as a "sign of weakness".
Senate Democratic leader Chuck Schumer said the president had "strengthened our adversaries while weakening our defenses and those of our allies".
Former CIA Director John Brennan described Mr Trump's actions as "nothing short of treasonous".
Ben Rhodes, who worked as deputy national security adviser under the Obama administration, tweeted: "Imagine working in the US intelligence community and watching the President of the United States attack you on foreign soil standing next to the President of Russia, a former intelligence operative."
As the press conference came to a close, CNN anchor Anderson Cooper said: "You have been watching perhaps one of the most disgraceful performances by an American president at a summit in front of a Russian leader, certainly that I've ever seen."
Here's some more of the political reaction from the United States: