Energy firm Cuadrilla has scaled back exploratory drilling in West Sussex on police advice, as more activists begin arriving at a nearby protest site.
The company has been looking for oil near the village of Balcombe, but has not ruled out using the controversial technique of fracking to release gas.
Police believe environmental activists at the site are about to begin a campaign of civil disobedience.
They expect about 1,000 extra people to join existing protesters this weekend.
The BBC's Angus Crawford said about 100 activists arrived by train at Balcombe railway station at about 13:00 BST before being escorted to the demonstration site by police.
Some of the protesters were waving anti-fracking banners and others were wearing Guy Fawkes masks, which have become a feature of demonstrations around the world.
Fashion designer Vivienne Westwood also joined campaigners on Friday afternoon.
Cuadrilla's decision to scale back its drilling means exploration operations will be effectively at a standstill just two weeks after drilling got under way at Balcombe.
The number of workers on the site has been reduced and large reinforced fences are being erected.
Cuadrilla said in a statement: "After taking advice from Sussex Police, Cuadrilla is scaling back operations ahead of this weekend's No Dash For Gas event.
"During this time, our main concern is the safety of our staff, Balcombe's residents and the protesters following threats of direct action against the exploration site.
"We will resume full operations as soon as it is safe to do so."
Matt Lambert, a spokesman for the energy firm, said workers had faced intimidation during their time at the site in Balcombe.
He added: "They're basically just oil men trying to go about their business, they're ordinary engineers, geologists and so forth, they're not used to this sort of thing and they've put up with it very stoically.
"It is quite unpleasant, people take photographs of you, take photographs of the number plate, we have to have police escorts out to get away from the site and of course it's unpleasant."
Campaigners welcomed Cuadrilla's decision to scale back its operations.
Luke Johnson said: "Cuadrilla's announcement that they'll halt drilling is already a victory for us, but it's only a start.
"We would like to make sure they don't frack in Balcombe, or anywhere else at all."
Cuadrilla is drilling a 3,000ft (900m) vertical well and a 2,500ft (750m) horizontal bore but said fracking for shale gas would need fresh permission.
'Take back power'
Environmental campaigners have been camped at the site for the past three weeks, with about 40 people arrested in that time.
Sussex Police believe more people will arrive for a six-day camp organised by the No Dash For Gas group.
It warned it would engage in mass civil disobedience - last year members occupied West Burton power station in Nottinghamshire.
Jamie Kelsey Fry, from No Dash For Gas, told BBC Radio 4's Today programme said that the protesters would be doing "everything they can to make the country think twice" about fracking.
"It was exactly these kinds of actions hundreds of years ago that gave women the vote with the Suffragettes. It's absolutely no different," he said.
Mr Kelsey Fry added the protesters would be risking "their liberty and personal harm" to highlight awareness of the government's "disastrous choice" to pursue fracking.
"This is not fun. People aren't going down there for fun. It's not fun to see the heavy policing that I predict will be happening over this weekend," he added.
In an open letter, Balcombe Parish Council chairman Alison Stevenson called on the group not to break the law.
Supt Lawrence Hobbs, of Sussex Police, said: "We are acutely aware of the impact that this is having on the residents of Balcombe and back their call to protesters not to engage in any criminal activity in the pursuit of their aims.
"We will continue to facilitate peaceful protest but newcomers to the site should be aware that if they commit criminal offences then we will collect the evidence and they will be arrested."
Cuadrilla has said it is "unlikely" to use the site for the production of fossil fuels.
Both Friends of the Earth and Greenpeace have spoken out against the company's activities.
Prime Minister David Cameron has urged the UK to embrace fracking - short for "hydraulic fracturing" - as an important source of energy production.
But opponents of the technique claim it can cause water contamination and environmental damage and can also trigger small earth tremors.
Professor Peter Styles, who advises the government on fracking safety, told the Today programme there was a danger that opposition to the process would "perpetuate the use of coal-fired energy... which is by far the worst kind of energy rather than gas".
"Replacing coal with gas is the logical step to reducing the UK's carbon footprint and having stable gas prices," he said.