Ineos confirmed the leak at its petrochemical plant in Grangemouth on Twitter, adding: "Our on-site responders are continuing to manage the incident with support from the emergency services."
It later said in a statement: "At noon we detected a leak on a pipeline inside our KG (Kinneil Gas) manufacturing plant.
"Purely as a precaution and to allow our response team to manage the situation, we have closed a number of access gates and are allowing only essential personnel to enter the south side of the site.
"All non-essential Ineos personnel who would normally be working in this area have been requested to report to their line manager."
Firefighters attended the scene to support the energy giant's on-site team, while the ambulance service sent specialist paramedics.
Police closed several roads around the site and urged motorists "to avoid the area and find alternative routes".
Schoolchildren were kept indoors during their lunch break as a precaution at the request of the police, and would be allowed to leave the premises as usual at home time, Falkirk Council said.
Falkirk resident David Golding tweeted: "Our kids in Grangemouth high and beancross primary being kept indoors over lunch."
Earlier this month, Ineos bought the Forties Pipeline System (FPS) for $250m (£193m).
The system carries the equivalent of 40% of UK North Sea oil production to the mainland every day - some 445,000 barrels - and nearly a third of its gas.
In September last year, the firm controversially imported shale gas to its Grangemouth plant on the Firth of Forth from the United States.
The first regular shipment to the UK from US shale fields, where the gas is produced using the controversial fracking method of blasting rock underground, was condemned by environmentalists.
But the company said it would preserve 10,000 jobs dependent on the Grangemouth plant, Scotland's largest industrial site.