After several "non-violent civil disobedience" protests in recent days, Extinction Rebellion group are set to attempt to "raise the bar" by blocking UK's busiest airport, which is expected to be used by nearly 250,000 people today.
Another officer is seen dragging a man sideways.
Police officers, who far outnumbered them, approached to warn them of potential arrest for trespassing.
A total of 718 people have been arrested and 28 people have been charged in relation to the protests, which have caused disruption for commuters in the British capital.
Despite the large police presence, the atmosphere remained calm, with demonstrators chanting "we love you" as officers advanced and formed a cordon around the truck. The bright pink Berta Cacares was finally carted away shortly after actress Dame Emma Thompson had climbed on top of it to deliver a speech during the protests.
With the pink boat no longer at Oxford Circus, supporters have sat down in a circle at the centre of the junction.
The activists had also blocked the Oxford Circus shopping district, but by 1700 GMT the roads had been cleared by police and traffic was flowing again.
Facing accusations that the demos were wasting valuable police time, Dame Emma told the Daily Mail: "We didn't ask for police resources, that is their decision".
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Local television footage showed a small plume of smoke near the ministry, and people climbing out the windows on a lower level. "We grabbed the children together into the safe room and just waited till the security forces arrived", she said .
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"However, we must engage with the urgency and seriousness of this - the only thing that matters".
Posting on Twitter, Extinction Rebellion Sheffield said "at least two" activists had been arrested and that the branch sent them its support.
Police stopped the youngsters from blocking the roads, forcing them onto the pavement.
"They made a decision to spend their money in this way, which I personally think is a waste of taxpayers' money".
Extinction Rebellion members, who have set up a garden on Waterloo Bridge, showed no signs of voluntarily shifting under the hot sun on Saturday, watched by tourists and families enjoying the Easter weekend.
Even bigger demonstrations are feared - with greater turnout prompted by the good weather and the airing of a David Attenborough documentary on climate change on Thursday.
The 16-year-old Swedish activist is due to meet senior British politicians next week having already met with Pope Francis and addressed the European Parliament.