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The 73-year-old condemned the outgoing president following the riots at the Capitol building, which Trump has been accused of inciting and which may see him become the first president to be impeached twice.
In a video shared on social media, Arnie said: "President Trump sought to overturn the result of an election - and of a fair election. He sought a coup by misleading people with lies.
"My father and our neighbours [in Nazi-occupied Austria during World War Two] were misled also with lies, and I know where such lies head.
"President Trump is a failed leader. He will go down in history as the worst president ever.
"The good thing is he will soon be as irrelevant as an old tweet."
That's a particularly savage burn, given that Trump has just been banned from his favourite social media site, Twitter.
He'd previously had tweets censored and a temporary suspension, but the company took the decision on Friday to ban him for good.
The official Twitter Safety account wrote: "After close review of recent Tweets from the @realDonaldTrump account and the context around them we have permanently suspended the account due to the risk of further incitement of violence.
"In the context of horrific events this week, we made it clear on Wednesday that additional violations of the Twitter Rules would potentially result in this very course of action.
"Our public interest framework exists to enable the public to hear from elected officials and world leaders directly. It is built on a principle that the people have a right to hold power to account in the open.
"However, we made it clear going back years that these accounts are not above our rules and cannot use Twitter to incite violence. We will continue to be transparent around our policies and their enforcement."
This is the most important moment in the history of social media.
The world's largest platforms have banned the world's biggest purveyor of lies, conspiracies and hate.
To every Facebook and Twitter employee, user and advocate who fought for this--the entire world thanks you! pic.twitter.com/jihFaOA39G
- Sacha Baron Cohen (@SachaBaronCohen) January 9, 2021
It comes after Mark Zuckerberg announced that Trump would be banned from Facebook and Instagram 'indefinitely'.
Zuckerberg said: "Over the last several years, we have allowed President Trump to use our platform consistent with our own rules, at times removing content or labelling his posts when they violate our policies.
"We did this because we believe that the public has a right to the broadest possible access to political speech, even controversial speech. But the current context is now fundamentally different, involving use of our platform to incite violent insurrection against a democratically elected government.
"We believe the risks of allowing the President to continue to use our service during this period are simply too great. Therefore, we are extending the block we have placed on his Facebook and Instagram accounts indefinitely and for at least the next two weeks until the peaceful transition of power is complete."
UK Health Secretary Matt Hancock said earlier today (10 January) that the move by social media companies 'raises questions about their editorial judgments and the way that they're regulated'.
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