Police investigating the London Bridge terror attacks which left seven people dead and dozens more injured have arrested 12 people.
They were detained after an armed raid in Barking, east London, the day after the van and knife rampage by three terrorists who were shot dead by police.
Scotland Yard said the investigation into the "horrific attack" at London Bridge and Borough Market was progressing rapidly with officers searching several addresses in the area.
The arrests in connection with the attacks came as Prime Minister Theresa May warned that Britain is in the grip of a spate of copycat terror plots and it emerged that an off-duty Metropolitan Police officer was among the 48 people injured by the terrorists.
The attackers brought carnage to the streets of the capital brandishing 12-inch knives, shouting "this is for Allah".
Police were called at around 10.08pm on Saturday to reports of a vehicle striking pedestrians on London Bridge.
The white van then continued to drive to Borough Market.
Three men then left the vehicle and began to stab and injure people, including a British Transport Police Officer and the Scotland Yard officer, neither of whom is believed to be in a life-threatening condition. A number of the 48 injured are suffering serious and life threatening injuries.
The suspects were confronted and shot by police at Borough Market within eight minutes of the first emergency call being made.
The trio were wearing what appeared to be explosive vests, with police later confirming they were fake. Mrs May said they were "worn only to spread panic and fear".
It was the third terrorist outrage to hit the country this year following the Westminster and Manchester attacks which all together have killed 34 people
As the Prime Minister delivered a stark assessment of the threat facing the UK President Donald Trump, in a series of early morning tweets, lashed out at London Mayor Sadiq Khan for his response to the attack, and said it is time to "stop being politically correct" about terrorism.
However, the president's criticism of Mr Khan for suggesting Londoners should not be "alarmed" was based on a clear misinterpretation of some of the mayor's comments.
Speaking outside Downing Street, Mrs May said that although there was no direct link between the three incidents: "As terrorism breeds terrorism and perpetrators are inspired to attack, not only on the basis of carefully constructed plots after years of planning and training, and not even as lone attackers radicalised online, but by copying one another and often using the crudest of means of attack."
The PM also:
:: Said defeating the "evil" ideology of Islamist extremism is "one of the great challenges of our time" but stressed it "cannot be defeated by military intervention alone".
:: Threw down the gauntlet to web giants and called for international agreements to regulate cyberspace, saying: "We cannot allow this ideology the safe space it needs to breed - yet that is precisely what the internet, and the big companies that provide internet-based services provide."
:: Set out her determination to stamp out "safe spaces" that exist in the real world, saying: "There is - to be frank - far too much tolerance of extremism in our country".
:: Said there was a need for Britain's counter-terror strategy to be reviewed to ensure police and security services have all the powers they need, while she also raised the prospect of longer jail terms for terror-related offences, including those which are "apparently less serious".
For the third time in a matter of weeks, counter-terrorism police units and security services have launched a huge investigation as they piece together the circumstances of the latest strike and its perpetrators.
Scotland Yard head Cressida Dick disclosed that police believe there were three attackers and that all had been "neutralised".
Ms Dick said: "We don't believe there is but we must make absolutely certain ... and as I said we have a very large investigation ongoing and we will be seeking to establish whether anyone else was working with or assisting in any way, or helping in the planning of this attack in the way that you would expect."
The Met Police commissioner declined to give any details about the identities of the attackers at this stage.
"Obviously a very high priority for us is to identify them, work out who they are, where they've came from, what is behind this," she said.
The official terror threat level in the UK is currently at severe, the second highest category, and Ms Dick said authorities are already at a "very high level of alertness".
Although police have "very good" resources and "extraordinarily highly-trained people" attacks are difficult to predict, the police chief added.
Giving an account of how the incident unfolded, Ms Dick said: "Our officers confronted the suspects and brought this terrible incident to a conclusion within eight minutes."
Witnesses said the terrorists drove into pedestrians on the bridge in the same way as Westminster Bridge attacker Khalid Masood.
They then emerged and attacked some of the people they had hit with the van before heading to Borough Market, where the pubs and restaurants were packed with Saturday night crowds, including many watching the Champions League final between Juventus and Real Madrid in Cardiff.
They were heard shouting "This is for Allah" as they attacked people.
One woman, who was in Black and Blue restaurant, said: "We just saw three guys come into the restaurant, stabbed someone in the face and someone in the stomach.
"One of them had a big knife. Then he came in and walked around the restaurant. I guess they just kind of stabbed anyone that they saw and knocked things on the ground and then we just hid."
Barman Alex Martinez was forced to take shelter in a bin when one knifeman entered El Pastor restaurant.
He said: "I saw that man with a knife in his hand and after that a man started screaming so I knew something wrong was happening.
"The man started screaming and dropping everything on the floor, the plates, the cutlery, the bottle, everything, he dropped it."