The Forth Road Bridge has been closed to traffic after 10 Greenpeace activists suspended themselves from the structure in order to block a tanker.
The group said it wanted to block an Ineos tanker carrying fracked gas to the Grangemouth industrial site further up the Firth of Forth.
Greenpeace said its international team wished to protest the production of plastics and the impact they have on the environment.
The activists suspended themselves using ropes and are about 25 metres from the water level at high tide.
They have a support team on the bridge itself and on the water, with the group saying they plan to stay in place for 24 hours â using hammocks to make themselves comfortable if they need to.
Greenpeace spokeswoman Amy Cameron told the PA news agency: âThe worldâs in the grip of a serious plastics pollution crisis, and in the UK the biggest plastic producer is Ineos.
âAt the Grangemouth site they produce enough plastic every day to make 60 million plastic bottles.â
They said petrochemical companies like Ineos are blocking progress for an international agreement on plastics.
She added: âWe are here today to say that if youâre going to try and block the plastics treaty, weâre going to block you.â
Ms Cameron said she expected the protest to be âsafe and peacefulâ, and precautions had been taken, adding: âFrom our perspective, we donât think thereâs any need for the police to be here.â
Asked about any disruption for travellers, she said: âItâs definitely not our intention to cause any disruption to any member of the public.
âOur climbers have access to bridge via the pedestrian walkway, which is separated from the roads â which is separated from the roads.
âSo there was no need to close the road bridge, if thatâs a decision that Police Scotland have taken, then thatâs their decision.â
Police Scotland said they were alerted at 1.05pm and officers were âengaged with those involvedâ. The public were asked to avoid the area.
The bridge is one of three crossing the Firth of Forth in central Scotland which links Edinburgh to Fife.
A spokesman for Ineos said: âTodayâs Greenpeace stunt is dangerous, disruptive and entirely counterproductive.
âIt may grab headlines but it does absolutely nothing to reduce plastic pollution or deliver real-world solutions, and ultimately threatens skilled jobs in Scotland.
âIneos produces materials that society relies on every day, from clean drinking water pipes and medical equipment to insulation, lightweight cars, and wind turbine blades â even the safety harnesses used by the protesters.
âThese arenât luxury items, theyâre essential to modern life and to lowering emissions.
âIn most cases, plastic is the most sustainable option available, confirmed by independent studies, which is why it has so many uses.â
The company said Greenpeace is attempting to block a shipment of ethane, which is used in plastic production.
The spokesman added: âUnless groups like Greenpeace stop attacking companies like Ineos and start supporting the rollout of waste infrastructure and advanced recycling, they are actively contributing to the problem.
âTheyâre standing in the way of the very solutions that are needed, whether we produce more or less plastic.â
