Special forces soldiers were deployed undercover in Northern Ireland.
At least eight of the covert units have been sent back to work with MI5 for the first time in four years as protests and violence escalate.
The move came after clashes in which dozens of officers were injured when stones, gasoline bombs and fireworks were hurled by youth gangs on Belfast’s nationalist Springfield Road.
MI5 and military intelligence officials fear the unrest, which has spread across the country, could trigger a return of deadly paramilitary attacks.
A team from the Special Reconnaissance Regiment will help coordinate intelligence operations and attempt to identify the leaders of the violence.
A source said: “Northern Ireland has been a tinderbox for a while. The tensions are extremely high.
“There are elements on both nationalist and loyalist sides who want British troops back on the streets, but that would be a disaster.”
The covert operation comes after more than a week of violent clashes sparked by post-Brexit tensions.
A mob was blown up with a water cannon on Thursday – the first time they had been used in Northern Ireland in six years.
Loyalists have urged their communities to refrain from protest activities out of respect for the Duke of Edinburgh.
But the riots continued over the weekend. Police were attacked with missiles and a car was set on fire in Tiger’s Bay, a loyalist area in north Belfast.
Some officers had thrown rocks and bottles at them and there were reports of gasoline bombs being used.
There was a heavy police presence in the area on Friday evening. Stones were reportedly thrown at officials in the nearby New Lodge nationalist area.
A witness said, “The worst thing I saw was a car that was stolen from a nearby street and set on fire. Then someone drove it and jumped out so the car could freely enter the police barricade. “
Nichola McKee Corner, whose journalist sister Lyra McKee was killed by the Real IRA two years ago, called for an end to the violence.
She said, “Destroying property and attacking people does nothing to advance any cause of any kind. In fact, it does the opposite. “
The news of the operation comes after the 23rd anniversary of the Good Friday Agreement.
Special Forces were withdrawn from Northern Ireland in 2011 but returned after attacks by Republicans in 2015. They were withdrawn in 2017.
The threat state in Northern Ireland is “serious”, the second highest, which means that an attack by Republican terrorists is “very likely”.
The threat level in the rest of the UK is “significant”, which means that an attack is likely.
The Ministry of Defense does not comment on special forces.