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 is protesting against the UK Government allowing 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 men and women, aged between 27 and 72, have been arrested on suspicion of aggravated trespass, and officers were working with partners to bring “a number of others” to safety.
By Monday afternoon, the force said it had made seven further arrests, all on suspicion of aggravated trespass, and that the site had been cleared of people.
Greenpeace said several protesters remained in place on Monday morning, with one activist on the offloading pipes, another hanging off the jetty and others occupying the jetty preventing the tanker from docking.
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: “The UK’s attachment to fossil fuels has backfired in the worst way possible – we’re funding a war, our energy bills and fuel costs are sky-high, and we’re driving the climate crisis.
“It has to stop.
“Putin invaded Ukraine nearly three months ago, and yet fossil fuel money from the UK is still funding his war chest.
“Ministers have kicked a ban on Russian oil imports to the end of the year despite strong public support for it.
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.”