A journalist was dragged out of the Trump-Putin news conference moments before the leaders were due to speak, after holding up a piece of paper bearing the words "nuclear weapon ban treaty".
Footage from the news conference showed the man - later identified as Sam Husseini - holding up the paper before being grabbed by a security guard.
The clip shows the guard, who was wearing an earpiece, trying to grab the paper off the man before taking him away.
Three other men helped escort the reporter away as the world's media took pictures and filmed the incident unfolding before them.
According to the Associated Press news agency, a US Secret Service agent spoke to Mr Husseini before Finnish security removed him.
He was sitting among US news media at the time of the incident and told journalists in the room he was a reporter from American magazine The Nation.
The magazine has since issued a statement, saying: "Sam Husseini, communications director of the Institute for Public Accuracy, received press accreditation from The Nation to cover the summit between President Donald Trump and Russian president Vladimir Putin in Helsinki.
"At a time when this administration consistently denigrates the media, we're troubled by reports that he was forcibly removed from the press conference before the two leaders began to take questions.
"This is a developing situation that we will be following closely."
A Twitter account that appears to belong to the journalist posted photos from the event, including images from within the press pack.
No posts have been published from the account since the incident.