LONDON, Dec 20: British police on Monday arrested 12 men on suspicion of plotting an act of terrorism inspired by Al Qaeda, in a major operation involving pre-dawn raids across the country.

The arrests of the men aged between 17 and 28 came amid heightened tensions following last week's Stockholm suicide bombing, which was conducted by a man who lived and was allegedly radicalised in Britain.

Five of the men held on Monday were arrested in the Welsh capital Cardiff, three were detained in the central English town of Stoke-on-Trent, one in England's second city Birmingham, and three were held in London.

Scotland Yard Assistant Commissioner John Yates, who is in charge of counter-terrorism policing, said it was a “large-scale, pre-planned and intelligence-led operation involving several forces”.

“With the current threat level of the UK at severe and with the information we had, I believe that today's arrests were absolutely necessary to keep the public safe,” he added in statement outside Scotland Yard headquarters.

A police statement said the men were held “on suspicion of the commission, preparation or instigation of an act of terrorism in the UK,” but Yates refused to give further details, saying the probe was in its early stages.

British media quoted sources as saying men were arrested because of fears they were linked to a potential bomb plot inspired by Al Qaeda, and that they had been monitored for months by police and the domestic spy agency MI5.

The BBC, quoting counter-terror sources, said the arrests were not believed to have been over a potential Mumbai-style gun attack, but a plot involving explosives or bombs instead.

The men were all arrested by unarmed officers at or near their homes at around 5:00 am (0500 GMT), apart from one suspect from Stoke who was detained at a house in Birmingham.

They are being held at police stations in central London, northwest England and the West Midlands. Police said searches were under way at their homes, the Birmingham address and another residence in London.

Reports said the five men held in Cardiff were all of Bangladeshi origin.

Britain, the former colonial power, had an ethnic Bangladeshi population of 283,000 at the last census, conducted in 2001.

Home Secretary Theresa May was fully briefed on the raids before they took place, the Home Office interior ministry confirmed.

Britain is on high alert after upgrading its perceived terror threat level earlier this year to “severe”, which is the second highest on a five-level grading and means a terrorist attack is “highly likely,” the Home Office says.

The threat level was hiked in January after a six-month spell at “substantial” — the only time it has dipped below the two highest levels since it was set up in 2006 following the London bombings in July 2005.

Those attacks on three Underground trains and a bus killed 52 people, plus the four suicide bombers, who were all British nationals.

Though they are not thought to be linked, Monday's arrests come shortly after the Dec 11 Stockholm bombing, Sweden's first ever suicide attack.

The bomber, Taimour Abdulwahab, was thought to have been radicalised in Britain, raising fresh concerns about Islamist militancy emanating from British shores.

Abdulwahab lived in Luton, north of London, with his wife and three children, after completing a sports psychology degree in the factory town.

That prompted Prime Minister David Cameron to admit that Britain had not done enough to counter domestic Islamic extremism.

The arrest also comes days after Interpol said it had received information from its office in Baghdad about possible Al Qaeda attacks on US and European targets.—AFP