Police have raided five homes as part of an anti-terror operation to foil a suspected Christmas terror plot.
Loud bangs were heard as an army bomb squad was deployed following a raid in Chesterfield and there were also operations by counter-terror officers in three parts of Sheffield.
Three men aged 22, 36 and 41 were detained at different addresses in Sheffield and a 31-year-old was held in Chesterfield.
The Independent understands the four men are suspected of preparing for an imminent terror attack, which would be the 10th thwarted by security services so far this year.
The unnamed men have been taken to a police station for questioning.
They were all arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
“The arrests were intelligence-led and preplanned as part of an ongoing investigation by Counter Terrorism Policing North East,” a spokesperson said.
“Three properties in Sheffield and one in Chesterfield are currently being searched in connection with these arrests.”
Police asked local people to “remain alert but not alarmed” as operations continued.
An area of Chesterfield was sealed off for much of Tuesday as an Army bomb disposal team worked with large numbers of police behind a cordon.
Local resident Simon Fox said: “We have seen Army, police from all over, we have seen a couple of packages being removed by the bomb disposal unit. You just don’t expect this to happen in such a small town.”
A woman who lives opposite the property that was raided said she and her neighbours were evacuated by armed police.
“It’s a Syrian family who lives there, they didn’t speak much,” she added. “[The man arrested] definitely had children, he had a beard, and he dressed trendy.
“He had two little kids, one was in a pushchair. His wife had a headscarf, they kept themselves to themselves, really.“
Meanwhile in the Meersbrook area of Sheffield, residents said they were woken by police blowing open the door of a run-down terraced house.
Joan Miller, 63, said she was woken by a “very loud bang” that shook her home at around 5.30am, adding: ”All sorts of different people have been living there.“
A few miles further north, in the Burngreave district, a mosque and community centre was sealed off by camouflaged officers with guns.
Two other raids are understood to have happened at a flat a short distance from the centre in Burngreave and a property in the town of Stocksbridge, just north of Sheffield.
It came after the director of MI5 confirmed that nine terror plots have been foiled since the Westminster attack in March.
Andrew Parker described the pace of the threat as “unrelenting” after revealing that 3,000 of the highest risk subjects are being monitored, alongside a wider pool of 20,000 others who have previously appeared on security services’ radar.
Terror attacks have left 36 victims dead and hundreds injured in the UK this year, with the national threat level remaining at severe.
Officials have warned the risk from both Isis, other Islamists and far-right extremists is growing amid a record number of referrals to the anti-terror Prevent programme.
The Government is currently drawing up a new counter-terror strategy due for release early next year.