Mock soldiers returned to the Irish border as protesters warned Theresa May not to allow a “nightmare” Brexit to derail Northern Ireland’s hard-won peace.

Theatrical demonstrators and Sinn Fein leader Mary Lou McDonald used sledgehammers to demolish a concrete wall erected near the frontier in Co Down on Saturday.

Fatigues-wearing “military personnel” looked on with mock machine guns raised.

Their protest comes as Ms McDonald told a Brexit conference that a hardening of the border in Ireland will be met with a demand for a unity referendum.

Speaking at the Beyond Brexit event in Belfast, Ms McDonald said crashing out of the EU with no deal in place would lead to a hardening of the border.

“There are no ‘little Irelanders’ here and we will not tolerate the narrowness of the Brexiteers or policy of isolation imposed by Brexiteers,” she said.

“A crash means a hardening of the border and the loss of rights and continued uncertainty and instability.

“A hardening of the border is inconceivable and will be met with the demand for a unity referendum.

“We don’t exactly know what will happen over the next few weeks or months. It’s not in our hands, it’s in the hands of a minority Tory Government in London, and that is the crux of the problem.

Sinn Fein president Mary Lou McDonald, left, and her deputy Michelle O’Neill arrive at the Beyond Brexit event. Credit: PA

Irish Taoiseach Leo Varadkar warned on Friday that troops could be reinstated at the border following a no-deal Brexit.

On Saturday, the colourful, costumed and placard-waving crowd from across Ireland rejected the “terrifying” prospect of a hard exit from Europe.

Pat Lambe, 58, said: “Land is sacred, Ireland is sacred to us and 20 years ago we all voted on the Good Friday Agreement, which to a lot of people said let’s move forward, there were compromises made on all sides.

An armed ‘soldier’ mans a lookout tower in the mocked-up border checkpoint. Credit: PA

“Let’s move forward and see, can we get peace and – relatively – we did get peace.

“There is a coach and horses being driven through that right now as regards the British Government and the arrogance of them.

“So the Good Friday Agreement means nothing?”

Security towers manned by the British Army in the hilly and remote area near the city of Newry were decommissioned in 2003 as it ended conflict-era operations in Northern Ireland in support of the police.

The protesters recreated them in a deeply political drama played out in front of masses of supporters.

Local man Aaron Crilly, 23, said: “Brexit scares me, it is a nightmare scenario.”

The protest saw actors in military fatigues playing the role of soldiers manning a wall at the border. Credit: PA

He was born in 1995, the year after the first IRA and loyalist ceasefires.

“I barely knew the Troubles but I grew up with the effects of the Troubles and my generation don’t want that to happen.

“We want to prevent that from happening ever again, but with Brexit it seems like there is a storm, that storm is coming back again and it is just terrifying.”

Protesters took turns using a sledgehammer to demolish the temporarily-erected concrete block wall.

Children write anti-Brexit messages on concrete blocks installed at the border. Credit: PA

Mr Crilly held his hammer high in mock triumph and brandished an EU flag.

An angle grinder was used to attack the metal structure housing actors playing soldiers, sparks flying.

They wore green and black fatigues and their faces were painted as they crouched in firing positions or aimed out of viewpoints from their “towers”.

If the UK leaves Europe without a deal, the free flow of goods could be disrupted by the creation of a hard frontier on the island, the European Commission has said.

Hundreds of people of all ages and from both sides of the border attended the protest. Credit: PA

Co Donegal demonstrator Tom Murray said: “All the peace and prosperity that we have enjoyed will be destroyed by a hard border.

“Communities could be dragged back into the old days of living in the shadow of someone else’s border.

“We are the people who will suffer the most.”