A top Facebook executive has said shadowy Russian-backed groups used the social networking platform to create mayhem during the 2016 US election, then set about raising doubts about the validity of Donald Trump's victory.

Key points: Social media giants are being grilled by US senators

Social media giants are being grilled by US senators Suspect Facebook accounts switched attention to sowing discord about Trump's election

Suspect Facebook accounts switched attention to sowing discord about Trump's election Al Franken asked why Russian money was still being accepted

While Mr Trump denies colluding with Russia, his campaign benefited from fake news and other malicious activity targeting his opponent Hillary Clinton.

But the treatment changed for the new President once he had found his way into the White House.

Facebook's General Counsel Colin Stretch told a US senate inquiry Mr Trump was soon being targeted by the same fake accounts that had been used to attack Mrs Clinton.

"We saw this concerted effort to sow division and discord," Mr Stretch told the Senate Judiciary Sub-Committee hearing.

"In the wake of the election and now President Trump's election, we saw a lot of activity directed at fomenting discord about the validity of his election.

"It continued until we disabled the accounts."

Tech giants Facebook, Twitter and Google have been called before the senate inquiry to face a grilling on how their platforms were used for Russian meddling in the 2016 election.

Mr Stretch confirmed Russian-backed actors were able to infiltrate Facebook in 2015 and spread misinformation until they were shut down several months ago.

"The foreign interference we saw is reprehensible," he told the inquiry.

"That foreign actors hiding behind fake accounts abused our platform and other internet services to try to sow division and discord.

"And to try to undermine the election is directly contrary to our values and goes against everything Facebook stands for."

Also appearing at the hearing, executives from other tech giants were forced to admit the vulnerability of their platforms to attack.

"The abuse of our platform to attempt state-sponsored manipulation of elections is a new challenge," said Sean Edgett from Twitter.

"At the time of the 2016 election, we observed instances — and acted on them — of automated and malicious activity.

"As we learned more about the scope of the broader problem, we resolved to strengthen our systems going forward."

"State-sponsored attackers are particularly pernicious. They are well-resourced. They are sophisticated. They are patient and often, by design, they are difficult to recognise," Google's Director of Law Enforcement and Information Security, Richard Salgado said.

At times senators were clearly exasperated with evidence from the tech giants as the executives were grilled on what effort the companies made to determine whether their customers were legitimate users.

Facebook's Mr Stretch admitted under persistent questioning that the company had no way of knowing the true identities of the 5 million advertisers who use the platform every month.

In another testy exchange, Democrat Senator Al Franken demanded to know why Facebook couldn't simply follow a money-trail in Russian currency.

"Those are two data points — American political ads and Russian money, rubles. How could you not connect those two dots?

"People are buying ads on your platform with rubles. They're political ads.

"You put billions of data points together all the time. That's what I hear that these platforms do. They're the most sophisticated things invented by man, ever. Google has all knowledge that man has ever developed. You can't put together rubles with a political ad and go like, 'Hmm, those two data points spell out something bad'?"

To which Mr Stretch answered: "Senator, it's a signal we should have been alert to and in hindsight it's one we missed."

The tech giants will continue giving evidence to the inquiry tomorrow.