Twelve men have been arrested during a major anti-terrorist operation, West Midlands Police said.
The men - five from Cardiff, four from Stoke-on-Trent and three from London - were detained on suspicion of the commission, preparation or instigation of an act of terrorism in the UK.
The suspects, aged between 17 and 28, were detained by unarmed officers about 0500 GMT.
Police are searching many properties, no explosives have yet been found.
West Midlands Police said in a statement: "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 will be held at police stations in central London, the North West and the West Midlands."
Most of the suspects are British but a small number are believed to be from Bangladesh.
Metropolitan Police Assistant Commissioner John Yates, national lead for counter-terrorism policing, said it was a "large-scale, pre-planned, intelligence-led" operation involving several forces.
"The operation is at its early stages so I am unable to go into any detail as I do not wish to say anything that may prejudice any future legal proceedings," he said.
"What I would say is with the current threat level in the UK at severe and with the information we had, I believe today's arrests were absolutely necessary in order to keep the public safe."
Mr Yates added it was "absolutely vital" the public remained vigilant and contacted the anti-terrorist hotline on 0800 789321 if they saw anything suspicious.
Home Secretary Theresa May said she had been kept fully informed of the police operation, adding the UK faced "a real and serious threat from terrorism".
Police sources told the BBC no firearms or explosives have been found so far during searches of the premises in Stoke, Cardiff, Birmingham and London.
Three terraced houses, including two neighbouring properties, were searched in Grove Street, Cobridge, Stoke-on-Trent. One of the suspects was arrested at a friend's house in Birmingham.
In Cardiff, police arrested three men at their homes in the Riverside area and two at their homes in the Ely area.
Cardiff councillor Ramesh Patel said police called him several hours after the arrests were made.
"It is a shock for me. Cardiff is not one of those places where you would expect this, although I realise it can happen anywhere," he said.
"I have to praise police for acting on the information they had very quickly and hopefully these are the right people they have arrested and the problem will be dealt with."
Police also arrested three men at their homes in central London, and it is understood they are being held at Paddington Green police station.
The BBC's security correspondent Frank Gardner said it was an MI5-led operation and the alleged plot was in "relatively early stages".
He said the police had up to 14 days to question the men before either charging them, releasing them or applying for more time.
He added there was always a balance to be struck between letting a plot run to build up enough evidence and protecting the public.
According to the Home Office:
- During 2009-10, 30% of terrorism arrests in England, Scotland and Wales resulted in individuals being charged
- Of these charges, 48% were for terrorism offences
- Over the same time period, 57% of those charged under terrorism legislation were convicted
- From 2006-10, only six people have been held for 28 days without being charged
The BBC's Danny Shaw said counter-terrorism sources had described the operation as significant and it was related to an investigation into al-Qaeda-inspired terrorism.
"The suspects are believed to have been involved in a plot against targets in the UK," he said. "This is not believed to have been a potential plot of a Mumbai-style attack, but a plot involving explosives or bombs."
Our home affairs correspondent said the officers who arrested the men were unarmed, suggesting the police felt there was no serious threat against them.
The alleged plot was not linked to the recent suicide bombing in Sweden, he added.
Earlier this month, Iraq-born Taimour Abdulwahab al-Abdaly blew himself up in the Swedish capital Stockholm. He had been living in Luton and had been thrown out of one of the town's mosques in 2007 for advocating violence, but he had not been reported to the authorities.
In September, intelligence sources said they had uncovered the early stages of an al-Qaeda plot to carry out co-ordinated attacks in the UK, France and Germany.
It was thought militants were planning to copy the 2008 attacks in the Indian city of Mumbai, where 10 gunmen went on a three-day rampage, killing 166 people.
The UK's terror threat level of "severe", the second-highest rating, means a terrorist attack is highly likely.