The Metropolitan Police has requested about 200 extra officers from neighbouring forces to help deal with the Extinction Rebellion protests in central London.
Since Monday, climate change demonstrators have occupied Oxford Circus, Waterloo Bridge and Parliament Square.
The Met says 715 people have been arrested and 28 have been charged.
Dozens of people are in Oxford Circus, despite being told by police to move.
The Met said it would continue to liaise with protesters and encouraged them to go to Marble Arch.
A statement added: “One thing that is unusual about this demonstration is the willingness of those participating to be arrested and also their lack of resistance to the arrests.”
It is understood the Met made a request to the National Police Coordination Centre (NPoCC) “late on Thursday” for help with extra officers from neighbouring regions in the east and south-east of England.
Essex Police and Kent Police confirmed they had sent officers to London under national mutual aid protocols.
A spokesman for the National Police Chief’s Council said “forces routinely share officers through mutual aid” in order to deal with large-scale events.
He added: “It is used to ensure an appropriate police presence exists where there is increased demand for it.
“NPoCC works with forces to determine their requirements should the need arise.”
On Good Friday, police removed a pink boat that had been parked in the middle of Oxford Circus since Monday.
Earlier that day, actress Dame Emma Thompson addressed demonstrators from the top of the ship.
On Saturday morning, about 30 protesters moved closer to each other and sat down in the centre of the Oxford Circus junction as police threatened more arrests.
One demonstrator said to the group: “Holding the space is important and being arrested is not undignified.
“We are here for an important reason, so we should be prepared to be removed for that. Being arrested is a statement.”
Members of the public watching have been asked to move on.
What is Extinction Rebellion?
Since the group was set up last year, members have shut bridges, poured buckets of fake blood outside Downing Street, blockaded the BBC and stripped semi-naked in Parliament.
It has three core demands: for the government to “tell the truth about climate change”; to reduce carbon emissions to zero by 2025; and to create a citizens’ assembly to oversee progress.
Controversially, the group is trying to get as many people arrested as possible.
But critics say they cause unnecessary disruption and waste police time when forces are already overstretched.