Art activists take on the Tate crowd over BP

The warmth of a real June day is still in the air. It is so soothingly hot, so summery, that it's hard to believe that activists are really about to take direct action outside the elegant confines of the BP and Tate Britain summer party.

The arriving partygoers are certainly taking a blitz-spirit attitude to the possibility, strolling through the groups of journalists and protesters and dismissing any suggestion that arts institutions like the Tate should not be taking money from fossil fuel companies like BP. "Utter nonsense," one partygoer says, robustly. "A bit silly, isn't it," suggests Alex Whitaker. "I'm not particularly pro BP," says Wendy Baron OBE (sometimes known as Mrs Sickert for her expert views on the artist), "but I am pro BP's art sponsorship. I think it's an excellent use of their money."

And then at quarter past seven, eight people dressed in black with veils over their heads walk slowly towards the Manton Street entrance. They are carrying black buckets with the BP logo painted clearly on them. It's clear that something odd is about to happen, but everyone is peculiarly frozen to the spot. Only when the first protester throws the contents of his bucket forward down the steps of the entrance do the assembled crowd realise that what is happening is molasses. About five gallons of it.

Others protesters from the art activist group Liberate Tate tip their buckets forward too, spilling the thick brown-black liquid across the pavement and down the steps to the party entrance (watch the video above). Then they pull feathers from their pockets and throw them on top of the molasses. The party guest beside me – a woman in turquoise separates - is beside herself with fury. As the protesters scatter through the dumbstruck crowd, she grabs at the veil on the head of one (moving a lot faster than any of the security guards, who seem to be processing events on dial-up rather than broadband) and pulls it off, shouting "How dare you! How dare you! Get him! Get him!"

One protester stays behind and adds sparkling confetti to the mess dripping slowly down the steps. "That's several thousand pounds worth of Portland stone ruined right there," says another partygoer. "They'll never get that out." Turquoise woman, who refuses to give her name, has got into an energetic argument with two activists who have been handing out leaflets. "What about working conditions in Colombia?" they argue. "Why should we be responsible for what happens over there?" she argues back. "That's life, isn't it?" Eventually she peels off to the gulag … sorry, Tate summer party, which is no doubt proceeding with added fizz inside.

The Tate's housekeeping team are meanwhile assembling their tools to attack the spill. Gilson Guimaraes (from Brazil) is standing, looking at the mess, while another member of his team starts using kitchen towels to daub, ineffectively, at the borders. "We'll use water and a vacuum cleaner," he says. A vacuum cleaner? In fact the Numatic that is rolled out turns out to be amazingly powerful and is soon slurping up the molasses with gusto.

"God, that's amazing. I might get one of those," murmurs an onlooker. Gilson is phlegmatic about the job. "It's not something you can do in five minutes," he says. "But we will get it done. For sure." Hmm. That's what BP thought.