The group is staging a four-day protest outside Parliament Square from Friday but has been in talks with the marathon race director to ensure minimal disruption.
A spokesperson for the group said: “We’re expecting around 40,000-50,000 people to be outside Parliament over the weekend.”
London Marathon director Hugh Brasher previously said his talks with Extinction Rebellion had prompted the agreement that “they will be uniquely asking all their participants to help guard the London Marathon”.
He said: “I trust Extinction Rebellion, they have been very clear of what they are doing and why. I hope Just Stop Oil and the other organisations listen to what we are saying.”
The environmental group’s spokeswoman said that while its stewards will protect the runners from the protest, “the whole thing has slightly been taken out of context”.
“We’ve been in close collaboration with the London Marathon since last year so that our protest can co-exist with the marathon,” she said.
“We met them a couple of days ago and said that we were going to protect people from the protest, and I think what the media has taken that to mean is that we’re going to protect the race from other groups protesting.
“Really what we meant by it was that we have stewards who will be helping to keep the area safe.”
She added that all the organisations signed up to take part in the protest have assured Extinction Rebellion they will not disrupt the marathon.
Just Stop Oil confirmed it had also been in talks with the marathon director, and activist Anna Holland told GB News: “I’m just going to say now: we’re not going to disrupt the marathon.
“We do also encourage anyone who’s coming to the marathon to also join us to march for the climate because it’s so important.”