Climate Camp Scotland says more than 100 people arrived from across Scotland at the plant, which is operated by Shell and Exxon Mobil.
The protest heard from speakers Maggie Chapman MSP, Linda Holt of the Mossmorran Action Group, local councillor Mary Lockhart, local activists Jemma Kettlewell and Bryce Goodall, and representatives of Glasgow Calls Out Polluters.
The protest formed part of Climate Camp Scotland’s Mossmorran Action Weekend, a weekend of camping, discussions and workshops near the petrochemical refinery, in collaboration with local group Actions Speak Louder Than Words.
The action follows campaigns by local residents, who have held weekly protests at the site gates, complaining of air and noise pollution from flaring.
Campaigners say Shell and Exxon have repeatedly breached environmental regulations at the site and are failing to align their business strategies with global climate targets.
Bryce Goodall, a resident of the area and campaigner with Actions Speak Louder than Words said: “The Mossmorran Action Weekend has been truly incredible.
"We have united people in our local community, from across Scotland, and internationally, in solidarity and support for climate justice.
"This weekend we listened to community concerns and empowered and skilled up our movement ready to mobilse further in achieving a just transition for the workers and communities around Mossmorran and internationally.”
Mim Black, an activist with Climate Camp Scotland said “Protest and dissent is vital to democracy, to challenging power and making the world a better place.
"After over a year in lockdown, the climate crisis has only gotten more drastic.
"This weekend was an opportunity to regroup and get ready for a huge year of climate action, because those in power seem unable or unwilling to do what is right. It is clear that Shell and Exxon cannot be trusted to put our wellbeing before shareholders profits.
"So by taking action, we are taking our collective wellbeing into our collective hands and we are proud to have run a successful, safe and inspiring weekend.”