Greenpeace says protesters have occupied a jetty where a tanker carrying a 33,000-tonne shipment of Russian diesel was due to berth, forcing it to turn around in the Thames.
The campaign group said 12 activists gained access to Navigator Terminals in Grays in Essex and climbed onto the jetty late on Sunday.
Greenpeace says the British government is allowing what it describes as “fossil fuel money” to flow to Russian President Vladimir Putin.
Essex Police said officers were called to reports of people gaining access to the terminal shortly after 11.05pm on Sunday.
The force said eight people have been arrested on suspicion of aggravated trespass, and officers are working with partners to bring “a number of others” to safety.
Greenpeace said several protesters remain in place, with one activist on the offloading pipes, another hanging off the jetty and others occupying the jetty preventing the tanker from docking.
They have unfurled a banner reading: “Oil fuels war”.
Greenpeace said the 183-metre-long vessel was due to offload at 11.59pm on Sunday.
Georgia Whitaker, oil and gas campaigner at Greenpeace UK, said: “Ministers have kicked a ban on Russian oil imports to the end of the year despite strong public support for it.
“To stand up to Putin, bring bills down and tackle climate change, the Prime Minister must get us off fossil fuels as fast as possible, stop ludicrous energy waste from our substandard draughty homes, and prioritise cheap, clean, homegrown renewable power.”
Greenpeace has tracked shipments of fossil fuels from Russia.
The Government has already banned Russian-owned, operated or flagged ships from entering the UK, but there is nothing stopping a vessel owned from elsewhere, from coming to the UK with Russian oil.
The tanker heading to the Essex terminal travels under a Greek flag, but is carrying fuels from the Russian port of Primorsk, Greenpeace said.
An Essex Police spokesman said officers are at Navigator Terminals and “working to resolve the situation quickly and safely”.
“Policing is not anti-protest, but we must intervene where there is a risk to life or where there is a suspicion laws are being broken,” the spokesman said.
“We are hoping this will be resolved safely and swiftly so that our officers can get back to protecting and serving the people of Essex.”
