Five men are being held under the Terrorism Act after being arrested close to the Sellafield nuclear site, police said today.
They were detained at 4.32pm yesterday following a stop check on a vehicle by officers from the Civil Nuclear Constabulary, who police the facility in West Cumbria.
The five, all in their 20s and from London, were arrested under the Terrorism Act, a spokesman for Cumbria Police said.
They were held in police custody overnight before being taken to Manchester this morning.
An investigation is now under way by the North West Counter Terrorism Unit.
A statement from Cumbria Police said: "At 4.32pm yesterday, Monday 2 May, police officers from the Civil Nuclear Constabulary conducted a stop check on a vehicle close to the Sellafield site in West Cumbria.
"As a result, police officers from Cumbria Constabulary arrested five men from London, all aged in their 20s, under Section 41 of the Terrorism Act.
"They were taken to police custody in Carlisle overnight and are being transported to Manchester this morning.
"The investigation is being led by the North West Counter Terrorism Unit.
"A road closure affected the area for a short period of time."
Both the location and timing of yesterday's incident will cause concern.
The terror arrests came outside the Sellafield site, which handles highly dangerous nuclear material.
And they were made in an apparent vehicle stop check within hours of the news breaking that Osama bin Laden had been killed.
At the same time, the public was being warned to remain extra vigilant for fear of a reprisal from groups sympathetic to al-Qa'ida.
A spokesman for Greater Manchester Police (GMP) said the investigation is in its early stages and no further information will be released yet.
A statement added: "At this stage we are not aware of any connection to recent events in Pakistan."
Section 41 of the Terrorism Act 2000 allows a police officer to arrest any person whom he "reasonably suspects" to be a terrorist.
The arrests are not believed to have been intelligence-led.
The suspects may have been taking photographs, a source suggested.
Scotland Yard counter-terror detectives are "making inquiries" to help GMP with its investigation.
The sprawling Sellafield site on the Cumbrian coast is heavily protected by both private security and officers from the Civil Nuclear Constabulary, some of whom are armed.
Sellafield is responsible for decommissioning and reprocessing nuclear waste and fuel manufacturing, on behalf of the Nuclear Decommissioning Authority.
The Sellafield site has been operational since the 1940s, when it was used as a Royal Ordnance factory supporting the war effort. The site is also home to the world's first commercial nuclear power station - Calder Hall, which operated from 1956 to 2003.
Today the site comprises a wide range of nuclear facilities, including redundant facilities associated with early defence work, as well as operating facilities associated with the Magnox reprocessing programme, the Thermal Oxide Reprocessing Plant (Thorp), the Sellafield Mox plant and a range of waste treatment plants.
Labour leader Ed Miliband declined to comment directly on the arrests but told reporters at an election press conference: "It is right that the Government has stepped up security at various places and obviously they will act on any intelligence they have."
Secret information revealed by WikiLeaks last week detailed threats from a terror suspect interrogated at Guantanamo Bay who spoke of al Qaida unleashing a "nuclear hellstorm" on the West if bin Laden was ever captured or killed.
A spokesman for GMP would not be drawn on whether the suspects were Asian or how close they were to the Sellafield site.
No further information from police is expected until officers from the counter-terrorism unit have questioned the suspects following their transfer to custody in Manchester.
According to reports last year, a counter-terrorism review of Britain's nuclear power plants was carried out after fears arose over safety at Sellafield.
Officials at HM Inspectorate of Constabulary, responsible for assessing the work of police forces across England, Wales and Northern Ireland, had begun an urgent assessment, it was claimed.
Concerns about protecting the plant may have come to light during an exercise in which special forces posed as terrorists to test security, according to a report in The Times, last December.
At that time no-one was available at the Civil Nuclear Constabulary or the Civil Nuclear Constabulary Authority to comment.
The Civil Nuclear Constabulary, which has a force of about 800 officers, most of whom are trained marksmen, is tasked with protecting the country's nuclear estate from the threat of international terrorism and has recently upgraded the quality and type of weapons and ammunition used by staff.