Greenpeace protesters blocked a jetty where a tanker carrying 33,000 tonnes of Russian diesel was due to berth, forcing it to turn around in the Thames.
The environmental group said 12 campaigners got in to the Navigator Terminals in Grays in Essex late on Sunday, and climbed on to the jetty.
They were protesting against the UK government allowing fossil fuel money to flow to Russian President Vladimir Putin, and fund invasion of Ukraine.
Essex Police said officers were called to reports of people gaining access to the terminal shortly after 11.05pm on Sunday.
The force said on Monday afternoon that 15 people had been arrested on suspicion of aggravated trespass, and that officers had cleared the site of protesters.
Greenpeace had previously said several protesters were blocking the offloading pipes, hanging off the jetty and others occupying the jetty to prevent the tanker from docking.
They unfurled a banner reading: “Oil fuels war”.
Greenpeace said the 183m-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.
“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 at Navigator Terminals had worked "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 always work to resolve incidents such as these as safely and swiftly as possible so that our officers can get back to protecting and serving the people of Essex."
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