Activists who aim to choke off the supply of petrol to south-east England have occupied the region’s busiest oil terminal.

Supporters of the Just Stop Oil campaign used telescopic ladders to climb over the fence at Navigator oil terminal in Thurrock, Essex, at 3am on Wednesday morning after trekking along the banks of the Thames to reach the rear of the site.

After using sleeping mats to avoid being impaled on the spiked fenceposts, they sprinted through the site to its loading bay, where fuel tankers are filled for deliveries to petrol stations across the south-east. Once there, they used lock-on devices and superglue to obstruct activities.

A source with the campaign said they believed the Navigator terminal to be the most important fuel distribution centre in the south-east, with up to 50% of the region’s vehicle fuels distributed from the site.

“Navigator is by far the busiest terminal in the south-east,” the activist told the Guardian. “Navigator, when it’s running, has about 60 tankers an hour going out.”

Police were removing some protesters by 7am. But six or seven had been able to clamber up into the pipework in the ceiling of the loading area, making their removal more difficult, Just Stop Oil’s activist said.

Just Stop Oil began taking action against oil terminals across the south and in the Midlands on Friday, at one stage blocking 10 in various locations. They are calling on the government to agree to a moratorium on new licences for fossil fuel projects and have vowed to disrupt fuel distribution until their demand is met.

Speaking from her lock-on position inside the terminal, one activist, named as Hannah from Brighton, said: “I’m 23 and the only way people will listen to me is if I lock on to the pipework of a fuelling station. Boris Johnson is signing a death sentence for my future by subsidising £25m a day of taxpayers money into new oil and gas.

“They can take away my liberty, they can take away my freedom and they can choose to ignore the alarm we are sounding on the climate crisis. But they can’t take away my courage to put my body on the line for every young person that is suffering from extreme heat in the global south or freezing to death in fuel poverty in the UK and having to choose between heating and eating.

“We are not going to die quietly. Please don’t be a bystander. Say no to new oil.”

Elsewhere, seven activists blocked one nearby roundabout that the campaign said was on a key route used by tankers accessing Navigator and other nearby oil terminals, while another 11 attempted to do the same elsewhere but were intercepted by police.

Just Stop Oil’s activists have targeted the Navigator site repeatedly since beginning their campaign on Friday, blockading the entrance to the facility and even digging a tunnel beneath its access road in an effort to stop vehicles leaving.

But an aggressive response by Essex police had forced the activists to change tactics. In recent days, van loads of protesters had been arrested on the way to the site, while a pair who had occupied the tunnels at the site had come out after police instructed firefighters to dig them out.

At the beginning of the campaign, Just Stop Oil’s activists had managed to blockade sites in London, Essex, Hampshire, Hertfordshire and the Midlands in the hope they could choke off the supply of fuel to the capital. Explaining the strategy, a campaign supporter said: “In the first 24 hours, we closed almost 100% of the south-east’s fuel supply. Petrol stations have three days’ fuel, so that means in 24 hours one-third will close, in 48 hours, two-thirds, and in 72 hours all of them will shut.

“That’s why in the first two days it was maximum intensity, to push it over the edge. We nearly pushed it over the edge and some started to close. Birmingham was targeted as that was viewed as the next place that would have the capacity to supply London.”

On Wednesday morning, after the activists invaded the Navigator site, the first police vehicle to arrive at the scene, inside the fenced off area, was a Welsh police van, suggesting that Essex police had called in mutual aid from other forces to tackle the threat posed by Just Stop Oil.

The Guardian has contacted Essex police for comment.