Two men have been arrested from Yorkshire on suspicion of terrorism offences.
The men, age 24 and 29, were arrested at separate locations in Huddersfield and Sheffield and searches were being carried out in properties in Huddersfield and Sheffield as well.
They were detained on suspicion of offences under Section 5 of the Terrorism Act.
The operation is based on recently received intelligence and is not connected to the terror attack in Manchester, the North Eastern Counter Terrorism Unit has confirmed.
Social media reports said residents living close to both locations reported being startled by large bangs and then seeing scores of police who confirmed the noises were in connection with the method of entry to the properties.
People living around a block of flats between Dun Street and Cornish Street, close to Sheffield's Inner Ring Road, said the bang went off at about 5.30pm Thursday evening.
One man who lived in the block said he saw a second-floor flat's door had been blown off.
Others said they saw armed plain-clothed officers in the triangular courtyard in the centre of the modern block.
The flats are fairly new, a short walk from the city centre, in an area which has seen a massive residential redevelopment over the past decade or so.
Cordons on the streets surrounding the property were lifted late last night, although a number of police officers continued to guard the block, which is opposite the Shalesmoor tram stop, as searches continued.
One stunned resident told how he heard a ‘huge bang’ as counter terrorism police raided the Sheffield flat.
Nick Meeks said he was working at his apartment on Cornish Street, in Kelham Island, when armed officers stormed the block of flats at around 5pm.
“I was sat there working on my laptop and there was a huge bang. I’ve never heard anything like it,” said the 27-year-old medical representative.
“Naturally I popped my head out of the door and there was smoke billowing down the corridor and police in full armed gear pointing guns down the corridor shouting at me to get back inside my flat. You do what you’re told when there’s a gun in your face.”
Mr Meeks said he was ordered to remain inside his second-floor flat at Daisy Spring Works for around half an hour before being allowed outside.
He told how the blast, which police said was down to the method used by officers to gain entry to the property, had caused extensive damage.
“There are cracks all the way down the corridor and huge cracks outside. It even blew a panel off the roof, whatever it was,” he said. “Within minutes of the bang loads of police had turned up and a cordon was set up. It was all very organised.”
Similar scenes were reported in Huddersfield, where a large number of armed police were deployed in the Ellison Street area of the town.
A spokesperson added: "The public may have heard loud bangs at the time that police entered the property. We would like to reassurance you that this was part of the method of entry to gain access to the property.
"We understand that people may have concerns as a result of this activity and we are grateful for the patience and understanding of the public while these enquiries are carried out."