Russian operatives, likely working from St Petersburg, provoked angry Americans to take to the streets, a US Senate committee heard on Wednesday.
The May 2016 protest, arranged by a group named Heart of Texas, was one example of Kremlin-backed efforts to destabilise the American electoral process.
Lawyers for three technology companies – Facebook, Twitter and Google – were told they were grossly underestimating the scale of the problem.
“You just don’t get it,” said California Senator Dianne Feinstein.
“What we’re talking about is a cataclysmic change. What we’re talking about is the beginning of cyber-warfare.”
She added: “We are not going to go away, gentlemen. This is a very big deal.”