Protesters have targeted a major conference on biofuels being held in Nottinghamshire.
Protesters claim the fuels are not as clean as hoped
Four thousand people have visited the event at the Newark Showground which discussed fuel made from crops, rather than fossil sources like oil.
But members of Biofuel Watch outside the venue claimed new evidence showed such fuels were not as clean as hoped.
Conference organisers said they wanted to see more debate about the the pros and cons of biofuels.
Biofuels are most often oilseed rape, wheat and palm oil and have been promoted as an environmentally-friendly alternative for drivers.
But doubts have been expressed over the effect of growing crops on the huge scale thought necessary and the quality of emissions
Dr Andrew Boswell, from Biofuels Watch, said: "When you look at the greenhouse gas balances with biofuels there is increasing evidence that actually the biofuels in lots of situation actually produce more greenhouse gases over the whole cycle than fossil fuel does."
Conference director Richard Price said he understood the need for discussion.
"I think we have some seriously conflicting messages coming across.
"I sat in today on the conference about food versus fuel - we had the green lobby there as well - and there are some genuine concerns about the greenness of biofuels.
"These things need to be aired we need to understand them."