Twelve men were arrested across Britain on Monday in counter-terrorism raids described by a senior police officer as essential to protect the public from the threat of an attack.
John Yates, Britain's most senior counter-terrorism officer, said the men were held on suspicion of the "commission, preparation, or instigation of an act of terrorism in the UK".
He described the operation to arrest the men as significant, but refused to give details of the allegations against them.
The BBC said in an unsourced report that the arrests were linked to an investigation into al-Qaeda inspired attacks within Britain.
The inquiry was led by the MI5 domestic security agency and the suspected plot was in its early stages, it said. Police would not comment on the report.
Yates said it was right to launch the raids, particularly because Britain is on the second highest state of alert for an attack. The threat level stands at "severe", meaning an attack is highly likely.
"With the current threat level at severe and with the information we had, I believe that today's arrests were absolutely necessary in order to keep the public safe," Yates said. "This operation has used significant resources from the country's counter-terrorism network."
The suspects, aged between 17 and 28, were mostly British, but also included a small number of Bangladeshis, the BBC said.
The 12 men were due to be questioned over the next few days. Five of them are from the Welsh capital, Cardiff; four are from Stoke-on-Trent in central England and three are from London.
A senior Iraqi official had said last week that he believed al-Qaeda was planning attacks in the United States, Britain and Europe around Christmas, one year after a failed attempt to bomb a US-bound airliner.
British police and security services have investigated dozens of suspected plots and arrested hundreds of suspects since the September 11, 2001 attacks on the United States.
Jonathan Evans, head of the MI5 security service, said in a rare speech last year that his biggest task was tackling al Qaeda-inspired militants. He highlighted a growing threat from militants in Yemen and Somalia.
Four suicide bombers killed 52 people on three trains and a bus in London in July 2005.
