Donald Trump has caused outrage in Washington after backing Russian President Vladimir Putin's denial that Moscow was involved in meddling in the 2016 US presidential election - over his own nation's intelligence reports
Mr Trump, who was seeking to change the relationship between the White House and the Kremlin, said he could think of "no reason" why Russia would be involved. There is consensus among US intelligence agencies that Russia did seek to alter the election, although Mr Putin has repeatedly denied it.
In Washington, the condemnation came thick and fast. Former CIA director John Brennan called Mr Trump's actions "treasonous", while Republican Senator John McCain called it “one of the most disgraceful performances by an American president in memory.”
The Arizona Republican said the summit with Russian President Vladimir Putin in Helsinki was “a tragic mistake.”
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The US president set off from Glasgow Prestwick Airport just after 4pm on Sunday, following his four-day visit to the UK.
He appeared to set the tone for the talks by congratulating Mr Putin and Russia for hosting "one of the best ever" World Cup tournaments.
Trump says he has 'low expectations' over Helsinki meeting with Putin
Senator Mark Warner says that he is worried Mr Putin will 'try to cut some deal, and frankly, take advantage' of Mr TrumpAsked what he would say to Putin, Trump said: “We’ll do just fine, thank you.”
Germany's foreign minister has said Europe can no longer rely on the US after Donald Trump called the European Union a "foe".
"We can no longer completely rely on the White House," Heiko Maas told the Funke newspaper group. "To maintain our partnership with the USA we must readjust it. The first clear consequence can only be that we need to align ourselves even more closely in Europe."
He added: "Europe must not let itself be divided however sharp the verbal attacks and absurd the tweets may be."
There is little hope for a breakthrough on the scale of Gerald Ford's meeting with Leonid Brezhnev or Bill Clinton's with Boris Yeltsin. It is thought likely that the meeting will produce a vague statement on Syria and a commitment to resume inter-governmental dialogue.
What we can expect from the Trump-Putin summit
Kremlin sees US president as 'a moron to take advantage of', believes one expertDmitry Peskov told broadcaster RT that the leaders had no strict agenda but claimed other countries should not be worried about decisions being made "over the heads of Europeans."
Mr Putin is expected to repeat denials of meddling in the 2016 US presidential campaign, according to Russian officials.
The Russian president was late for meetings with the Queen, Pope Francis and German chancellor Angela Merkel, among many others.
They are expected to continue their discussion over lunch in the Hall of Mirrors before taking questions at a joint news conference, scheduled for around 2.50pm.
Meanwhile, the Russian Foreign Ministry has replied to Mr Trump's tweet blaming US "stupidity and foolishness" for bad relations with Russia: "We agree."
First of all Mr President I’d like to congratulate you on a really great World Cup. One of the best ever from what everybody tells me and also for your team, itself, doing so well.
I watched quite a bit and in the United States we call it soccer and I watched quite a bit of it and I watched the entire final and the semi-finals and they were really spectacular games, but it was beautifully done so congratulations on that.
Most importantly we have a lot of good things to talk about ... we have discussions on everything from trade to military, to missiles, to nuclear, to China, we’ll be talking a little bit about China – our mutual friend President Xi.
I think we have great opportunities together as two countries that frankly we have not been getting along very well for the last number of years. I’ve been here not too long but it is getting close to two years, but I think we will end up having an extraordinary relationship. I’ve been saying, and I’m sure you’ve heard, over the years ... that getting along with Russia is a good thing not a bad thing.
I really think the world wants to see us get along. We are the two great nuclear powers. We have 90% of the nuclear – and that’s not a good thing it’s a bad thing. I think we can hopefully do something about that because it is not a positive force it is a negative force so we’ll be talking about that among other things.
And with that the world awaits and I look forward to our personal discussion which I think begins now and then we are going to meet our whole team. You have quite a few representatives as I do. We all have a lot of questions and hopefully, we will come up with answers most importantly. It is great to be with you.
The Russian president spoke briefly during the short joint appearance before the media, occasionally nodding as he slouched in his chair.
A US official told CNN that it was to "develop a leader-to-leader relationship", prevent leaks of sensitive information and stop aides interrupting him during the meeting.
“Distinguished president, I am delighted to be able to meet you here in Finland. We have been in continual contact by telephone since the last time, and obviously the time has come to have a business-like conversation because there is so much happening in the world that we need to talk about.”
The parallel meeting is their first since Mr Pompeo's appointment.
Democrat Adam Schiff explains his reasoning below, while Democrat Mark Warner had previously warned that Mr Putin could "take advantage" of the US president given the lack of oversight of the meeting.
Billboards have been running with the phrase "Mr President, welcome to the land of the free press".
In the last Reporters Without Borders World Press freedom report Finland ranked at number 4, the US ranked at 45.
Some context for our UK readers, Britain placed at 40th.
At the city's Senate Square, about 20 male "pregnant Trumps" and 20 women wearing pink gags railed against a Trump order reinstating what critics call a "global gag rule" that bans providing federal money to international family-planning groups that perform abortions or provide information about them.
Another march that drew hundreds of people was a mishmash of messages — some supporting German Chancellor Angela Merkel and some shouting "Fascists go home!" Some marchers questioned the legitimacy of Trump's election victory since he lost the popular vote. Another sign read: "A woman's place is in the Resistance."
"I don't think the two people meeting today are going to care whether we are here or not, frankly," Helsinki protester Tina Aspiala, told the Associated Press. "It's more of a global solidarity against a lot of things that are messed up, and I think that that's why it's worth coming out."
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Outside the Helsinki palace where the meeting took place hundreds of protesters gathered in the streets in support of a variety of causes including abortion rights, gay rights and anti-facism. One sign read: "Lets make human rights great again".
The US president's summit with Mr Putin follows contentious visits to Nato in Brussels and to the UK, where he was greeted with widespread demonstrations against his administration.
Mr Putin meanwhile is on a high after Russia's triumphant hosting of the World Cup, although questions linger over the novichok poisoning of Sergei and Yulia Skripal on British soil.
In an interview on Good Morning Britain before heading off to Helsinki, Mr Trump described his Russian counterpart as a "competitor" and said it was too early to say whether they were friends or enemies.
He also claimed that the EU was one of America's biggest enemies because of "what they do to us in trade" and repeated his description of the media as the "enemy of the people".
Mr Trump went on to issue a tweet blaming "US foolishness and stupidity" and what he called the "rigged witch hunt" for bad relations with Russia, to which the Russian Foreign Ministry replied "We agree".
He had been due to meet alone with the Russian president for one-and-a-half hours from around 11am UK time, but Mr Putin arrived at Helsinki airport half an hour later than expected. The joint press conference was due to start at 2.50pm.
The US president has said he was going into the meeting with "low expectations". There is no official agenda for the talks and Democrats have raised fears that Mr Putin will "try to take advantage", such as by reaching an agreement on the annexation of Crimea.
