About 170 workers have staged an unofficial walkout from the Exxonmobil petrochemical plant at Mossmorran in Fife over working conditions.
Union GMB said the workers, who walked out on Wednesday had "continuously raised their concerns about conditions and safety on-site".
Their employer Bilfinger said the unrest had started with other contractors on the site.
The plant is currently undergoing a restart after closing down in August.
Exxonmobil said in a statement: "The safety and security of our employees, neighbours and others is our top priority."
There were reports of huge flaring on Thursday night with residents making a flood of complaints to environment watchdog Sepa.
A GMB Scotland spokesman said: "Relations between the workforce and management have been deteriorating for some time.
"Workers have continuously raised their concerns about conditions and safety on-site but have frankly been ignored.
"You can't operate a major gas terminal this way and it shouldn't be beyond the capabilities of Exxon and their sub-contractors to ensure workers feel safe and valued."
Bob MacGregor, Unite regional industrial officer, said: "Unite has just become aware of the situation involving a number of contract workers at the Mossmorran ExxonMobil site.
"Cleary there are significant issues which have resulted in a number of workers walking off the site, which the company needs to resolve."
James Glen, chair of Mossmorran Action Group said: "How can communities have confidence that Mossmorran is safe when 100 workers have staged a walkout in part over health and safety concerns at the ethylene plant?
"On the day when Sepa gives Exxon a poor rating on its 2018 performance, workers take industrial action over safety concerns and the operator responsible announces it will boycott a public meeting with regulators for a third time, it is hugely disappointing that environment minister Roseanna Cunningham says she is too busy to come to Lochgelly on Friday to attend the same public meeting."
However a spokesman for Bilfinger UK, denied it was to do with safety concerns.
He said: "Workers employed by a separate contractor at the Fife Ethylene Plant in Mossmorran staged an unofficial industrial action on Monday, which a number of our employees joined with unofficial sympathy action. This action resulted in a dispute over payment for the time our employees had withdrawn their labour.
"We are now working closely with our employees and the trade union, Unite, to quickly resolve this dispute.
"There is no dispute relating to working conditions, welfare, redundancy measures and health and safety between Bilfinger employees working at the plant and Bilfinger UK. We maintain an open dialogue with our employees and have rigorous health and safety processes in place, with no incidents reported since the contract began in September 2019."
He added: "Our continued focus is on the ongoing success of our operations at the plant, safeguarding the long-term employment of our skilled and experienced workforce."
Green MSP Mark Ruskell said: "The Mossmorran workers are absolutely right to take a stand. This comes on a day when Sepa, which monitors the risk of pollution or environmental damage, has rated the site 'poor'.
"Mossmorran is an industrial relic and the frontline of our climate emergency challenge. Not only has the site been causing misery for the local community with unplanned flaring, now we hear staff are concerned about safety.
"This community faces an uncertain future, with no attempt to build sustainable alternative jobs in the area.
"Neither ministers or operators have engaged enough with this community."