Workers at Harland and Wolff have closed the shipyard's gates as part of a protest following news that the business is up for sale.
They have demanded Boris Johnson's government renationalise the yard and saves their jobs.
The protest began on Monday afternoon and has continued into the night.
The Unite union said workers decided to take this action ahead of the expected arrival of administrators on Wednesday.
A short time ago, a shipping container was being brought to the front gates of Harland and Wolff for shelter for workers who say they will protest all day and all night until a solution is found to save their jobs... pic.twitter.com/GyX3B48OEb— Maria McCann (@BBCmariamc) July 29, 2019
A spokesman for the Norweigian parent company of Harland and Wolff said the company had no comment to make at this stage.
The business has been up for sale amid serious financial problems at its Norwegian parent company.
The yard employs around 130 people, specialising in energy and marine engineering projects.
There is a large banner on one of the two iconic cranes saying "Save Our Yard".
Unite's regional coordinating officer Susan Fitzgerald said: "Workers fighting to preserve Harland and Wolff deserve the support of everyone concerned for the future of jobs and skills in Northern Ireland.
"Unite is again calling on his (Boris Johnson's) government to nationalise Harland and Wolff and cooperate with the workers to chart a way forward for the yard."
Workers at Harland and Wolff are demanding the Government re-nationalise their yard. They have closed the gates and say they won't leave until a solution is found. pic.twitter.com/2JaVOX8JEN— Dan Stanton (@DanStantonrep) July 29, 2019
GMB regional organiser Michael Mulholland said the Scottish government is drawing up plans to nationalise the Ferguson shipyard on the Clyde, and said it would be "ironic" if one of Boris Johnson's first acts as prime minister was to let Harland and Wolff "sink by default".
A steel worker at the yard, Joe Passmore, said workers had "never thought they would be put in this position".
"It was always assumed the management and politicians would come up with a plan but so far they've failed, but we aren't prepared to see this place fail when we know it can be viable and vibrant," he added.
Trade unions have been hoping that the yard could benefit from plans to build more Royal Navy ships in the UK.
But now there is a risk it will not survive for long enough to benefit.
Senior politicians and Invest NI have held a series of meetings with Harland and Wolff's management and unions in recent weeks.
Harland and Wolff is one of Northern Ireland's best known businesses and its two huge cranes dominate the east Belfast skyline.
It was founded in 1861 and at the height of production during World War Two employed around 35,000 people.
Its best known vessel is the Titanic, which was built at the yard between 1909 and 1911.
It last built a ship, the Anvil Point, in 2003.