Ellen Barry, Moscow correspondent for The New York Times, is tweeting on behalf of Human Rights Watch's Tanya Lokshina, who is attending a meeting with Edward Snowden.
More top news
One person has been arrested and no security personnel were injured, the Secret Service said.
It is the fifth publicly known guilty plea in the probe into possible collusion between the Trump campaign and Russia during the election.
Louella Eve Fletcher-Michie, daughter of Holby City actor John Mitchie, died last year at the Bestival music festival in Dorset.