Beekeepers and their supporters have gathered in Parliament Square to urge the Government to support an EU-wide ban on certain pesticides.
The demonstration came ahead of a vote in Brussels on Monday which will decide whether Europe introduces a two-year moratorium on various neonicotinoid pesticides.
One of the organisers, Matt Shardlow, chief executive of nature conservation organisation Buglife, said: "Britain abstained last time and has made no commitment this time, but we want them to support a ban across Europe. Some 73% of the British public support a ban on these insecticides, we want the Government to follow their lead."
He said that even if the vote was lost, he was hopeful that there may be other ways forward as the European Commission has a legal responsibility to protect the environment.
One of the protesters, biological research graduate Robert Mitton, 28, from Ealing, west London, said: "They started using these pesticides in the 90s. Since then there has been a rapid decline in the abundance and diversity of bee species globally.
"There is a mounting body of scientific evidence that these pesticides are having sub lethal effects and in effect making the bees sick. They can make them forget things, such as which flowers are rewarding to them, and impair their ability to reproduce, affecting their long-term survival."
Beekeepers dressed in their protective costumes for the protest and many protesters wore brightly coloured striped clothing. Other groups involved with organising the event included Friends of the Earth, Greenpeace, Pesticide Action Network UK, RSPB, and the Soil Association.
Organisers said dress designers Vivienne Westwood and Katharine Hamnett had handed a petition at 10 Downing Street in support of their aims.
Friends of the Earth's head of campaigns Andrew Pendleton said: "Ministers can't ignore the growing scientific evidence linking neonicotinoid insecticides to bee decline. Their claims to be concerned about bee health will ring hollow if they fail to back European moves to restrict the use of these chemicals."
A Defra spokeswoman said later: "As the proposal currently stands we could not support an outright ban. We have always been clear that a healthy bee population is our top priority, that's why decisions need to be taken using the best possible scientific evidence and we want to work with the Commission to achieve this. Any action taken must be proportionate and not have any unforeseen knock-on effects."