There will be a heightened police presence at British ports and major events over the coming days in response to the massacres in Paris, a senior Metropolitan Police officer has said.
Mark Rowley, the assistant commissioner for specialist operations in London, said the UK's official terror threat level was already at "highly likely" - the second highest it can be - and he expected the Cobra meeting called by Prime Minister David Cameron today to consider raising it further.
He called for "vigilance" from the public, stressing that the fight against terrorism is dependent on maintaining the trust and confidence of the UK's communities to provide police with information about suspicious behaviour.
He said: "We can't let the terrorists defeat us by becoming fearful and withdrawing from the streets.
"It comes down to trust between communities and the ability for the public to trust the police and work with us and supply us with the sort of information they have been very effectively over the past year."
Mr Rowley told BBC Radio 4's Today programme: "We are strengthening our policing stance across the country at the moment, and we are reviewing that later this morning, no doubt, with the Prime Minister at Cobra.
"We are strengthening our ability to protect the public. We will be looking at events today and the policing profile there. We've made some small changes.
"People may notice a little change at ports. People may notice some changes at events in big cities across the country - extra police officers, extra checks, extra vigilance from the police.
"We will constantly keep that under review, and there are many events over the coming days and weeks we will have to look at."
Mr Rowley, who is national police lead for counter-terrorism, added: "We are doing things like strengthening our policing of ports, we have been strengthening policing on the streets. We have put out some guidance across the country.
"Our liaison officer in Paris is seeing what we can find out there and see if there is anything to feed into our own intelligence picture."
British police had already been preparing for the possibility of an attack of the kind seen in Paris, said Mr Rowley.
"It is one of the scenarios we had been thinking about," he said. "In July this year we did a massive exercise in London for exactly this type of scenario - a multiplicity of firearms attacks.
"We have been planning for it. It is very much on the radar in terms of something we think may happen, but of course we constantly hope it won't, and all police and intelligence agencies across the Western world are trying to defeat it, but we know that there are people out there trying to do that."
Mr Rowley said the current threat level of "severe", which indicates that an international terrorist attack is highly likely, is "the second highest level it can be and the highest it can effectively be for a sustained period".
He added: "However strong we are, however ready we are, however many people we are arresting, we are dependent on the confidence and the strength of communities to work with us."
Mr Rowley said: "This takes us back to not letting terrorists defeat us. They are trying to sow discord, they are trying to sow fear, they are trying to divide communities.
"At the heart of the success of countering terrorism - regardless of all the operations we run and all the arrests we try to make - is the relationship between the public and the police and it needs to be stronger than ever.
"We have had more reports than ever before of behaviour of concern and individuals of concern over the last year or so and we need that to continue."
He said: "It is that vigilance which gives us the first insight, particularly when we have a terrorist organisation which has a range of methods, from the ghastly mass killings that we have seen in Paris through to provoking over the internet vulnerable young people and people with mental health issues to do lone wolf-style attacks."