Just Stop Oil has claimed 13 of its members have been arrested while protesting ahead of the coronation.
The eco-activists say the members were handcuffed by The Mall on Saturday morning. A spokesperson for the organisation said a further five demonstrators were arrested at Downing Street.
The group representative said their plan was “only to display T-shirts and flags”, adding: “This is a dystopian nightmare.”
Just Stop Oil have caused mayhem in recent months by blocking roads and disrupting sporting events.
It comes after protestors from anti-monarchist group Republic were also arrested during demonstrations on Saturday.
Footage on social media appeared to show Republic CEO Graham Smith being apprehended by police on St Martin’s Lane in Westminster.
Police and the intelligence services are on red alert to prevent disruption from protesters, with more than 11,500 officers deployed to ensure a “spectacular celebration”.
Armed police, sniffer dogs, mounted units and marine officers on the River Thames are among the specialist teams that are being deployed.
Around 100 heads of states are expected to be among the 2,200 guests at Westminster Abbey.
Scotland Yard warned those who break the law should expect “very swift action” from officers who will have “an extremely low threshold” to intervene.
Deputy Assistant Commissioner Ade Adelekan said: “What we will not stand for is anyone committing criminal acts in the name of protest.
“We will come down very swiftly to make sure people going about their normal business, who just want to enjoy the Coronation, are not interfered with.”
DAC Adelekan confirmed that holding a placard up during the procession does not constitute “serious disruption”. But if holding up a sign was to cause a disturbance in the crowd, it could lead to police intervening, he said.
A “ring of steel” has been formed around Westminster Abbey with roads closed off to counter threats.
