Workers from plane manufacturer Bombardier are pressing the Government to take urgent action to help secure their jobs in the face of mounting concerns over a trade dispute with the United States.
They unfurled a huge banner opposite Parliament and will meet MPs to warn of the impact on jobs across the UK if the dispute is not resolved.
Bombardier, which employs more than 4,000 workers in Belfast, has been hit by a proposed 80% levy on exports following complaints by Boeing that the Canadian-owned company had dumped its C Series jets at "absurdly low" prices.
Business Secretary Greg Clark has told MPs the complaint brought by Boeing is completely unjustified and the UK will do everything it can to see the issue resolved.
As part of those efforts, on Tuesday evening Prime Minister Theresa May stressed in a phone call with US President Donald Trump "the importance of the jobs provided by the Bombardier factory to the people and economy of Northern Ireland".
But Steve Turner, assistant general secretary of the Unite union, said phone calls were not enough.
Joining a group of workers from Belfast, he said the levy was part of Mr Trump's America First policy.
The Government has a number of levers it could use to resolve this quickly, especially as the Ministry of Defence has £4.6 billion worth of contracts with Boeing.
"This is the kind of situation we might be facing in the future, with free trade not fair trade, and the Government needs to act now."
John Clarke, who works at the Belfast plant, said: "People are worried about the future and everyone feels uncomfortable about what might happen.
"We are here to raise awareness of the situation and stress to MPs and the Government the urgency of the situation."
Unite wants Mrs May to summon Boeing to a summit with Canadian Prime Minister Justin Trudeau and workforce representatives.
Mr Turner added: "The British Government has a duty to defend UK manufacturing jobs against the bullying behaviour of Boeing.
"A failure to do so will signal that any ambition ministers have for a coherent industrial strategy is effectively in tatters and that they are happy to put Trump's America First policy ahead of UK manufacturing jobs.
"Boeing's case is without merit, a fact that Prime Minster Theresa May has herself admitted. Theresa May and her Government need to be battling for Northern Ireland's Bombardier workforce which makes some of the most technologically advanced wings in the world.
"The UK Government must heed the call of Bombardier's Northern Ireland workers and summon Boeing to an urgent summit involving Prime Minister Theresa May and the Canadian Prime Minister, Justin Trudeau, and trade unions.
"Ministers should leave Boeing in no doubt that a failure to do so will lead to sanctions on current and future work on behalf of the UK Government."