Counter-terrorism officers secretly recorded an alleged Isil-inspired terror cell in Barking last month discussing how to use YouTube to plot a van and knife attack in London, The Daily Telegraph can reveal.The investigators were monitoring the alleged extremist cell in the east London borough weeks before Saturday night’s attack in the capital, which left seven dead and 48 injured. The Islamic State group has claimed responsibility for the attack.On Sunday police arrested 12 people, among them seven women – all of them in Barking and other parts of east London, where at least one of Saturday’s killers was believed to have lived. Those arrested were aged from 19 to 60.Three terrorists shouting “This is for Allah” as they stabbed at their victims were killed by police just eight minutes after the alarm was raised shortly after 10pm on Saturday. Officers revealed that they fired an “unprecedented” 50 bullets at the attackers, who wore fake suicide belts to maximise panic and fear. Now The Daily Telegraph has learnt that police had been monitoring an extremist cell in that area since March.Theresa May declared “enough is enough” as she set out her plan to tackle terrorism. “We cannot and must not pretend that things can continue as they are,” she said.Addressing the nation from outside Downing Street after the second terrorist attack during the election campaign, the Prime Minister said that internet companies had allowed terror to thrive by creating a “safe space” for extremism and called for international agreements to “regulate cyberspace”. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Mrs May said Britain had been too tolerant of Islamist extremism, allowing copycat killers to repeat atrocities in the wake of the Westminster and Manchester attacks.An unnamed friend of one of the terrorists claimed he had reported the man to the police anti-terrorist hotline after he became radicalised by watching extremist videos on YouTube. He said the attacker had listened to hate speeches by the infamous American Islamist Ahmad Musa Jibril.A neighbour of one of the suspected attackers claimed she had reported him to Barking police two years ago, after he began “brainwashing” her children at a local park. She said she had confronted him after her two children came home and said “Mummy I want to become a Muslim.”In other developments: Stories of heroism during the terrorist attacks emerged, including a policeman who was stabbed in the face and head as he tackled the killers armed only with his baton; Donald Trump said it was time to “stop being politically correct” about terrorism; The first victim of the attack was named as Canadian Chrissy Archibald, who had moved to Europe to be with her fiancé Police disclosed they had accidentally shot and injured a member of the public as they fired on the terrorists; It emerged Islamic State of Iraq and the Levant (Isil) had urged followers to ramp up attacks during the holy month of Ramadan, which finishes on June 24; A tribute concert in Manchester for the victims of last month’s suicide bombing went ahead as planned.Police confirmed that three men drove at speed across London Bridge in a hired van, knocking down pedestrians, before leaving the vehicle at nearby Borough Market and marauding through bars and restaurants stabbing and slashing indiscriminately at victims. One of the terrorists reportedly shouted “this is for my family, this is for Islam” as he plunged a knife into a 23-year-old victim who is now in hospital. In the recent surveillance operation in Barking last month one suspected jihadist discussed what appears to be an identical plot. Election campaigning by the Conservatives and Labour was suspended for most of Sunday as a mark of respect but resumed in the evening and Thursday’s polling will go ahead as planned.Mrs May said “violence can never be allowed to disrupt the democratic process”. Security at polling stations is expected to be increased on Thursday, and Metropolitan Police Assistant Commissioner Mark Rowley said there would be “increased physical measures” on London’s bridges to protect the public.The Prime Minister is expected to chair a meeting of the Government’s Cobra emergency committee this morning. The terrorist threat level remains at severe. The Daily Telegraph has discovered that one alleged suspect said the intended method was to “use a car as a weapon” and boasted that he had radicalised more than a dozen “students” in Barking “wanting to martyr themselves”.He said the plot would involve driving at pedestrians and then getting out of the vehicle to attack others. He added: “YouTube videos all make it properly easy to do.”One of the plotters talked about “getting an automatic [vehicle] so the boys can drive it”.A neighbour of one of the men shot dead by police on Saturday said he had asked him about where he could hire an automatic van, during a conversation when the neighbour was using a hire van to move furniture. Material connected to the alleged plotters who were being tracked last month included pages from the Islamist magazine Ramiyah, which suggest that easy “prey” include “a drunken kafir [non-believer]”.In a statement released 24 hours after the killings on its official Amaq media channel, Isil said the three suspects had been part of a “sleeper cell”.It said: “A security unit of Islamic State fighters carried out the London attacks yesterday.” It is the third attack in Britain the terror group has claimed in as many months. Isil had called on its supporters in Europe to carry out attacks on “infidels” during the Muslim holy month of Ramadan. Armed police stand guard near Borough MarketCredit:Andy Rain /EPA “Our society should continue to function in accordance with our values. We must come together, we must pull together, and united we will take on and defeat our enemies.” The Joint Terrorism Analysis Centre decided not to increase it to critical, the highest level, which happened after the Manchester suicide bombing last month. Amber Rudd, the Home Secretary, explained that “we don’t believe there are additional elements which could be carrying on the attack”, in other words the police believe all members of the cell are accounted for.Mrs May praised the “courage and great speed” with which the emergency services responded. She said: “On behalf of the people of London and on behalf of the whole country, I want to thank and pay tribute to the professionalism and bravery of the police and the emergency services, and the courage of members of the public who defended themselves and others from the attackers.”Mrs May said the country was “experiencing a new trend in the threat we face, 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”.She went on: “Everybody needs to go about their lives as they normally would. We are all shocked and angry today – but this is our city. We will never let these cowards win and we will never be cowed by terrorism. pic.twitter.com/kcosumkdCR— Sadiq Khan (@SadiqKhan) June 4, 2017 Theresa May said that internet companies had allowed terror to thriveCredit:KEVIN COOMBS/Reuters Sadiq Khan, the Mayor of London, told Londoners that there would be extra police officers on the streets but that they should not be “alarmed”.President Trump appeared to have misinterpreted his advice, saying on Twitter: “We must stop being politically correct and get down to the business of security for our people. If we don’t get smart it will only get worse.“At least seven dead and 48 wounded in terror attack and Mayor of London says there is ‘no reason to be alarmed!’” The men under surveillance are also said to have talked about attacking a bridge – in their case Westminster Bridge – and going to the gym to make their arms more powerful and therefore more lethal when using knives. Neighbours said the London Bridge attack suspect was a keen gym user.