There are no words to describe the heart-breaking scenes in Manchester, following after the most barbaric terrorist attack on the United Kingdom since the 7/7 bombings on London’s transport network in 2005. Innocent children and young people were indiscriminately slaughtered just after they had enjoyed watching their heroine, Ariana Grande, perform at the Manchester Arena. One of the victims, named Saffie Roussos, was just eight years old.

This was a truly evil act of terrorism, carried out by a coward, in the name of a perverse and twisted ideology. Our grief at what has happened is matched only by our absolute determination to ensure that cowards like Salman Abedi — and the sick ideology that he followed — will never be allowed to win. Londoners, Mancunians and Britons across the United Kingdom will never, ever be cowed by terrorism.

Anyone who doubted our resolve just needed to witness the vigil organised in Albert Square in Manchester by Andy Burnham, the Mayor of Greater Manchester. Thousands of Mancunians came together — in their grief and heartbreak — to show that their great and historic city will never be broken by terrorism. They prayed together. They chanted “Manchester” together at the top of their voices. And they listened together to the defiant words of Tony Walsh’s love poem to Manchester, This is the Place: ‘And these hard times again, in these streets of our city, but we won’t take defeat and we don’t want your pity. Because this is a place where we stand strong together, with a smile on our face, Greater Manchester forever.’

Once again, the world witnessed the heroism of our emergency responders and bystanders in the face of the most terrifying of situations — from the police and security services who are still working around the clock, doing extra shifts with all hands on deck to chase down every lead in this investigation, to the ambulance and National Health Service staff in Manchester who helped the injured throughout the night of the attack.

Some of the stories we have heard are inspiring. From the ordinary Mancunians — parents waiting for their children rushing to help others when they heard the blast — to hoteliers who opened their doors to the injured — to taxi drivers who drove from Liverpool when they heard the news to help others get home.

As the Mayor of London, my highest priority is keeping Londoners and visitors to our city safe from harm. I have been in constant contact with the Metropolitan police, the mayor of Greater Manchester, the government and others as the investigation into the Manchester attack continues and we review our security preparedness across our city and country.

On Tuesday night, it was announced that the national threat level was increasing to “critical” on the advice of security experts. This is the very highest level of security preparedness we have in the UK. As a result, there will be more police on London’s streets over the coming days — including armed police at key sites and busy stations. Military personnel will also be deployed around London as part of this, codenamed Operation Temperer.

Under the direction of the Met they will guard some key sites such as parliament, in order to free up armed police officers to focus on other parts of London.

I would urge all Londoners and visitors to remain calm and vigilant as the investigation continues. The best thing we can do is show that it is business as usual. Our emergency services prepare day in and day out for situations like this. Their plans are well rehearsed, well prepared and well coordinated. Nonetheless, if you do see anything suspicious please report it to the police.

Londoners understand better than most how the people of Manchester will be feeling today. It is just over two months since the horrific attack on Westminster, in which police constable Keith Palmer and four innocent bystanders were brutally murdered and more than 50 injured. The attack in Manchester happened four years to the day since the cowardly murder of fusilier Lee Rigby in Woolwich. And of course we will always remember the tragic events of 7/7, in which 52 people were killed and more than 700 injured.

In the face of such horrifying and tragic events, Londoners have never given in to those who seek to destroy our way of life and divide our communities — nor will the people of Manchester now. Our great cities and our great nation stands more united than ever before. And more determined to tackle the perverse and twisted ideology behind those who have carried out these atrocities.

We must now redouble our efforts to destroy the poisonous ideology held by these extremists and terrorists to stop another incident like this happening again. We must renew and improve the government’s anti-extremism programmes, so that they are more effective at rooting out home-grown extremism. We must build stronger and more resilient communities that can resist the extremists. And we must give our police and security services the resources they need in order to keep us safe.

In the aftermath of the Westminster attack, we showed the world what it means to be a Londoner. Today, the people of Manchester are showing exactly what it means to be a Mancunian. When we are faced with adversity, when we are tested on who we are and what we stand for, we always pull together. We stand strong for our values and our way of life. We always have, and I know, we always will. We will not be defeated.

— Guardian News & Media Ltd