US President Donald Trump has arrived home from his one-on-one meeting with Vladimir Putin in Helsinki to a wave of criticism that has crossed party lines.

Mr Trump has been slammed for the comments he made at a press conference after the two leaders had met, in which he appeared to take the side of Mr Putin and Russia over his own FBI in regard to Russia's alleged meddling in the 2016 presidential election.

Standing alongside Mr Putin, Mr Trump steered clear of any confrontation with the Russian, going so far as to question American intelligence and last week's federal indictments that accused 12 Russians of hacking into Democratic email accounts to hurt Hillary Clinton in 2016.

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"I have great confidence in my intelligence people, but I will tell you that President Putin was extremely strong and powerful in his denial today," Mr Trump said.

"He just said 'it's not Russia'. I will say this — I don't see any reason why it would be."

His scepticism drew a quick formal statement — almost a rebuttal — from Mr Trump's director of national intelligence, Dan Coats.

"We have been clear in our assessments of Russian meddling in the 2016 election and their ongoing, pervasive efforts to undermine our democracy, and we will continue to provide unvarnished and objective intelligence in support of our national security," Mr Coats said.

The fallout to the meeting and press conference has called the conduct of both Mr Trump and Mr Putin into question, with the Russian President again denying the Russian state had any part in influencing the US election — even as a Fox News journalist tried to hand him the indictment documents of 12 Russian intelligence officials during an interview.

Politicians from both major parties and former intelligence officials appeared shocked, dismayed and uneasy with Mr Trump's suggestion he believes Mr Putin's denial of interfering in the 2016 elections.

Almost 24 hours after the summit, Mr Trump reiterated his positive opinion of the meeting in Helsinki on Twitter.

"While I had a great meeting with NATO, raising vast amounts of money, I had an even better meeting with Vladimir Putin of Russia. Sadly, it is not being reported that way — the Fake News is going Crazy!" Mr Trump said.

Sorry, this video has expired US media try to give Putin copy of Mueller indictments

It was a remarkable break with US intelligence officials and the Justice Department and, just as alarming for some, Mr Trump also put the two countries on the same footing when casting blame for their strained relations.

Senator John McCain called it, "one of the most disgraceful performances by an American president in memory", while Senator Ben Sasse called it "bizarre".

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Senator Jeff Flake called the press conference "shameful" and Senator Lindsey Graham tweeted that it was a "bad day for the US".

"This was a very good day for President Putin," said Senator Bob Corker, the chairman of the Foreign Relations Committee. He said Mr Trump's refusal to condemn Russian interference in the 2016 election made the US, "look like a pushover".

Republican Will Hurd said he had seen Russian intelligence manipulate many people in his earlier career as a CIA officer. But, he tweeted:

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House Speaker Paul Ryan weighed in to say there was "no question" Russia interfered in the 2016 presidential election and "no moral equivalence" between the US and Russia.

"The President must appreciate that Russia is not our ally," Mr Ryan said in a statement.

"[Russia] remains hostile to our most basic values and ideals."

Putin 'is gloating' after Helsinki press conference

Professor Tim Nichols, an expert on Russian affairs at the US Naval War College, said America's opponents will now think they have "carte blanche" to do as they please, without consequences.

"Putin in particular is gloating, and thinks that he can pretty much do whatever he wants and that the President, when faced with a difference of opinion between what the Russians are doing and what his advisers are telling him, will cover Putin's back," he said.

Sorry, this video has expired Putin says he wanted Trump to win the election

Professor Nichols said Mr Putin is now feeling "emboldened" and he thinks the Russian President could now start causing problems on NATO's eastern border and continue unchecked with military actions in Syria.

"And when confronted, he'll shrug and say, well, you know, the President of the United States believes me."

Much of the Republican rebuke came from politicians who have been willing to openly criticise the President, a group that remains a minority in the GOP.

Many top Republicans remained on the sidelines after the Justice Department on Friday indicted 12 Russian intelligence officials for election-related hacking.

But several Republicans who do not typically buck the President raised concerns, shocked by Monday's performance.

Trump ally Newt Gingrich called it, "the most serious mistake" of Mr Trump's presidency — and one that, "must be corrected immediately".

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Democrats pleaded with their GOP colleagues who have majority control of Congress to rein in the President and become a stronger legislative check on the executive branch.

Senator Chuck Schumer, the minority leader, said never in the history of the country had an American president supported an adversary the way Mr Trump sided with Mr Putin.

Sorry, this video has expired Putin says he wanted Trump to win the election (Photo: AP/Pablo Martinez Monsivais)

He challenged Republicans to move beyond words and to confront the President directly by increasing sanctions on Russia and requesting testimony about the summit from Trump administration officials, among other things.

"We need our Republican colleagues to stand up for the good of this country," he said.

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And House Minority Leader Nancy Pelosi said Mr Trump's weakness in front of Mr Putin was not just "embarrassing" but also, "proves that the Russians have something on the President, personally, financially or politically".

Republicans have been hesitant to fully confront the President, who remains popular among GOP voters back home. But Mr Trump's hold on the GOP is being put to the test by his willingness to align with Mr Putin, a leader whom Republicans routinely describe as an enemy of the United States.

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ABC/AP