Police arrested 12 men in early morning raids on three cities Monday aimed at thwarting a major new terrorism plot against British targets.

Assistant commissioner John Yates, Britain's senior counterterrorism police officer, said the suspects were detained in London, in the central English city of Stoke-on-Trent and in Cardiff, Wales.

''The operation is in its early stages so we are unable to go into detail at this time about the suspected offenses,'' Yates said in a statement. ''However, I believe it was necessary at this time to take action in order to ensure public safety.''

Police confirmed that the men were detained by unarmed officers - indicating that any attack was unlikely to have been imminent.

The arrests follow several weeks of surveillance and are not believed to be linked to the suicide bombing in Sweden on December 11, or to recent concerns aired by European and US officials over purported plots to carry out Mumbai-style commando attacks on European cities.

Police said 11 of the suspects were arrested at or close to their homes at around 5am, while the remaining man was detained at a property in the central English city of Birmingham.

Five of the arrested men are from Cardiff, four from Stoke-on-Trent and three from London, police said.

Officers said the suspects range in age from 17 to 28 and will be questioned on suspicion of commissioning, preparing or instigating acts of terrorism.

The arrests are the most high-profile antiterror raids in Britain since April 2009, when 12 men were detained in raids across northern England. All were released without charge, but authorities insisted they had thwarted a major al Qaeda bomb plot in the northern city of Manchester.

Yates said the latest arrests followed close coordination by officers from several different cities. ''This is a large scale, pre-planned and intelligence-led operation involving several forces,'' he said.

A British security official, speaking on condition of anonymity because of the sensitive nature of his work, said the arrests did not relate to any planned holiday season attack. Iraqi officials had claimed last week that captured insurgents believed the Stockholm bombing was part of a series of planned al Qaeda attacks against the US and Europe during the Christmas season.

Those claims were rejected by both British and German officials, who insisted there are no specific threats to their countries over the festive period.

In October, the US State Department advised American citizens living or travelling in Europe to be wary amid reports that terrorists were planning attacks on a European city.