Hundreds more police officers are being posted across the UK after the nation raised its terrorism threat level to 'severe'.
Key London landmarks and shopping areas will be guarded by officers on Wednesday night as one of the nation's top police chiefs warns people to be alert.
Police are expecting thousands of people to congregate ahead of a second national lockdown starting on Thursday.
The move has been prompted by terrorist attacks across Europe, which saw four people shot dead by an Islamist extremist in Vienna on the eve of Austria's national lockdown on Monday.
Last week, three people were killed at a church in Nice, France, by an ISIS supporter.
“I know Londoners will be anxious following the truly awful attacks across Europe in recent weeks and our hearts are with all those affected," Scotland Yard's Deputy Assistant Commissioner Matt Twist said on Wednesday.
"Last night it was announced that the threat level has been increased to ‘severe,’ this means an attack is ‘highly likely’.
“I would ask Londoners to not be alarmed by this, but to remain vigilant. If you see or hear something that doesn’t feel right to you, let us know.
“In London tonight, we will have a number of policing resources on duty to look out for you and keep you safe. We will be working closely with British Transport Police and City of London Police to provide a heightened policing presence in key areas, such as high streets but also at transport hubs."
Last-minute shopping before stores close has seen popular streets, such as Oxford Street, become busier than usual.
Famous landmarks, such as Buckingham Palace and the Houses of Parliament, which have been the locations of previous terrorist attacks, are expected to have many more officers present.
Pub chain Greene King closed some of its London pubs at 3pm on Wednesday ahead of the 10pm deadline due to the heightened terror risk.
In the northern city of Manchester similar alerts have been issued.
Manchester Arena was the scene of an ISIS atrocity in 2017.
"In light of the change to the threat level, we are reviewing our operational plans with a view to enhancing protection and preparedness, so members of the public may notice an increased police presence over the coming days," Assistant Chief Constable of Greater Manchester Police, Nick Bailey said.
The UK terrorism chief, Assistant Commissioner Neil Basu, said although there is no evidence to link the European attacks to the UK, the terrorist threat level has been raised as a precautionary measure.
"The threat level from international terrorism has changed from 'substantial' to 'severe' as a precautionary measure in response to the events in France and Austria," he said.
"This threat level moving to 'severe' means it is highly likely that a terrorist attack could happen in the UK, but I want to stress that the change is not based on a specific threat.
"However, I do urge the public to remain vigilant and to report any suspicious activity to police. Now, more than ever, we need communities to stand together and reject those who seek to sow division and hatred between us.
"We need communities and families to bring to our attention anyone they perceive may be vulnerable, a danger or escalating towards terrorism."
"We will begin to increase our levels of visible patrols and implement other security and protection measures," he added.
"We are also working closely with our local communities – businesses, faith groups and community groups across the UK to provide reassurance and seek their support and assistance in keeping the UK safe.
"Our efforts can only succeed where we have the support of the public – it is communities standing together that defeats terrorism."
