Demonstrators have thrown an oil-like substance and feathers on the entrance to Tate Britain in London in protest at its acceptance of BP sponsorship.

The group, calling itself The Good Crude Britannia, is calling for the gallery to sever ties with the company over the Gulf of Mexico oil spill.

Last night the gallery threw a summer party, partly to mark 20 years of support from the oil giant.

More than 170 artists have written an open letter protesting the sponsorship.

The poured substance was believed to be molasses Q&A: Why is arts sponsorship important?

Printed in Monday's Guardian, the letter called the BP logo "a stain on the Tate's international reputation".

Playwright Caryl Churchill, cartoonist Martin Rowson and comedian Robert Newman were among the 171 signatories.

Others included artist John Keane and electronic musician Matthew Herbert, who appeared at the Glastonbury festival last weekend.

Jane Trowell, of environmental arts campaign group Platform, said: "BP is trying to repair its tarnished reputation and buy our approval by associating itself with culturally important institutions like Tate."

She added that she hoped it would soon be "socially unacceptable" for cultural institutions to accept funding from oil companies.

Even a 'property' with as high a profile as the Turner Prize or the Edinburgh Comedy Awards struggles to find and retain sponsors Will Gompertz, BBC arts editor Read Will Gompertz's blog

But in a statement, Tate Britain defended the deal saying BP's support "has been instrumental in helping Tate develop access to the Tate Collection and to present changing displays of work by a wide range of artists in the national collection of British art".

Last week the Tate joined other institutions that receive BP sponsorship in thanking the oil company for its support.

"We are grateful to BP for their long-term commitment," said the Tate, the Royal Opera House, the British Museum and the National Portrait Gallery in a joint statement.

"The income generated through corporate partnerships is vital to the mixed economy of successful arts organisations and enables each of us to deliver a rich and vibrant cultural programme."

Those sentiments have since been echoed by art historian Sir Christopher Frayling, former chairman of Arts Council England.

"Now is not the time to get squeamishy about sponsorship," he said on Radio 4's Today programme.

"The system is utterly dependent on sponsorship from companies and large firms."

Yesterday's open letter, however, claimed sponsorship from the likes of BP and Shell was a "means by which attention can be distracted from their impacts on human rights, the environment and the global climate".

"These relationships enable big oil companies to mask the environmentally destructive nature of their activities with the social legitimacy that is associated with such high-profile cultural associations," the letter reads.