Officers were called to the scene in St Clements Way, in West Thurrock Grays at around 8pm on Sunday after reports of a group obstructing the road and a tanker.
One person climbed on top of the vehicle while others are reported to have deflated its tires.
The road was temporarily closed but re-opened at around 3am on Monday.
Following a busy week of protest activity, our priorities remain to keep Essex moving, keep people safe and minimise the disruption to the public
Ass Ch Con Glen Pavelin
Eight people remain in custody after being arrested on suspicion of offences including going equipped to cause criminal damage.
Environmental campaign group Just Stop Oil reported a number of “ordinary people” had stopped the tanker in support of its cause.
Just Stop Oil revealed on Tuesday protesters had dug a tunnel underneath the carriageway – a key delivery route for a nearby oil terminal.
Since then, it claims those inside the tunnel have broken through to the road surface.
A second tunnel has been created under nearby Stoneness Road, which has been closed.
According to its website, the activist group works to “demand that the government immediately halt all future licensing and consents for the exploration, development and production of fossil fuels in the UK”.
Assistant Chief Constable Glen Pavelin said: “Following a busy week of protest activity, our priorities remain to keep Essex moving, keep people safe and minimise the disruption to the public.
“Two tunnels on Stoneness Road and St Clements Way remain occupied. The road at St Clements Way remains partially open, and Thurrock Council Highways Department are ensuring an overview of the road.
“We appreciate that this situation is incredibly frustrating for the public, but we ask drivers to check your journey before you travel in the West Thurrock area.”
Ass Ch Con Pavelin added officers were continuing to engage with the group and invited them to start a conversation to find another way for them to protest.
He said: “We take the concerns of this particular protest group its members and supporters seriously and willing to work with you so that your rights to peaceful protest can take place without members breaking the law.”