Counter-terrorism detectives today arrested 12 people amid fears of a Christmas bomb attack in the UK.

The men, aged between 17 and 28, were held in Birmingham, Cardiff, London and Stoke-on-Trent on suspicion of the commission, preparation or instigation of an act of terrorism in the UK, police said.

Searches at several properties began after the arrests, with detectives and forensic experts looking for any scientific evidence of materials that could be used to make explosives.

The counter-terrorism operation targeting some of those arrested had been under way for some time, and is described as "significant". At least some of those arrested are believed to have been under surveillance.

The raids were launched "to take action in order to ensure public safety", the country's leading anti-terrorism officer, Assistant Commissioner John Yates, of the Metropolitan police, said.

"This is a large-scale, pre-planned and intelligence-led operation involving several forces," Yates said. "The operation is in its early stages, so we are unable to go into detail at this time about the suspected offences."

The home secretary, Theresa May, said: "I have been kept fully informed about the police operation that has resulted in 12 arrests. For obvious reasons it is not appropriate for me to comment further at this early stage of what could be a complex and lengthy investigation."

"We know we face a real and serious threat from terrorism and I would like to thank the police and security service for working to keep our country safe."

The closeness of Christmas appears to have been a factor, but it is not clear whether investigators were nervous because of any specific intelligence that an attack was imminent, or whether the "landmark" date made them anxious.

Last year, al-Qaida attempted a Christmas Day attack on a plane travelling to Detroit in the US.

The arrests followed a long term undercover investigation led by MI5, according to counter-terrorism officials.

Those arrested were alleged to be involved in a serious plot, according to officials, indicating this was more than an operation designed only to disrupt or warn off suspects. Well-placed officials described the investigation as significant. Sources pointed to the balance MI5 and the police have to make between the potential danger to the public and a need to get sufficient intelligence and evidence to stand up in court. Also taken into account were the resources taken up in an investigation involving tracking the movements of 12 people.

Of those arrested today, five - aged 23, 23, 25, 26 and 28 - are from Cardiff, while three, aged 17, 20 and 28, are from London.

Four of the men - two 26-year-olds, a 19-year-old and a 25-year-old - are from Stoke. All 12 were arrested under the 2000 Terrorism Act. A number of the men are Bangladeshi. Intelligence was being gathered on the men until the arrests were made at 5am. Police said counter-terrorism officers were unarmed when they detained them.

The man arrested in Birmingham was detained by officers in the Small Heath area, and is believed to have recently moved there from Stoke. The arrests in London were made in Newham and Tower Hamlets, in the east of the capital.

Of the five men arrested in Cardiff, three were said to be from Riverside, near the Millennium stadium. Two are believed to be from Ely, a mile west of the centre. All are believed to be in their 20s.

City councillor Ramesh Patel said he understood they were members of the Bangladeshi community. "The police told me and other councillors that they were going to make these arrests," he said. "It's a shock for the community. The police seem to have acted quickly, but it's important not to accuse these people yet. It will be up to the police and courts."

West Midlands police said: "The suspects were detained by unarmed officers at approximately 5am. All were arrested at or near their home addresses, with the exception of one suspect from Stoke, who was at a domestic property in Birmingham. Searches are now being conducted at the home addresses, plus the address in Birmingham and another residence in London."

The suspects are being held at police stations in central London, the north-west and the West Midlands, police said.

The arrests were co-ordinated by the West Midlands counter-terrorism unit and also involved the Metropolitan police's counter-terrorism command, South Wales police and Staffordshire police. The South Wales and Staffordshire forces said they had no further details about the operation to give.

The government's assessment of the risk of a terrorist attack in the UK remains at "severe" – the second-highest level – meaning an incident is seen as highly likely.

In October, the US state department issued a travel alert for Europe, saying groups linked to al-Qaida could be planning attacks. This followed speculation that al-Qaida was planning a "commando-style" attack similar to the 2008 Mumbai massacre, in which 166 people were killed.

Although police and security services have been planning for the possibility of such an attack, it is understood that the arrests are not connected with a possible Mumbai-style event. They are also not connected with the attempted suicide bomb attack in Sweden last week, which was carried out by a man who had lived in Luton.

British counter-terrorism sources counselled caution about claims by the Iraqi government that al-Qaida supporters are planning concerted suicide attacks in the US and Europe during the Christmas season. Iraqi interior minister Jawad al-Bolani said last week that the failed bombing in central Stockholm last weekend was among alleged plots revealed by captured insurgents. Most recent reports of terror threats were not credible, well-placed British officials said.