Police numbers have been stepped up at key locations around the UK in the wake of the Brussels attacks.
The country’s most senior counter-terrorism officer said the move was a precaution.
Assistant Commissioner Mark Rowley, of the Metropolitan Police, said: “Our thoughts are with the people of Brussels following this morning’s horrific attacks.
“As a precaution forces across the UK have increased policing presence at key locations, including transport hubs, to protect the public and provide reassurance.
“This is not in relation to any specific information or intelligence.”
In London, additional officers have been mobilised, he added
London Mayor Boris Johnson told Sky News: “I want to stress we have no intelligence that suggests there’s any imminent plan against this city. We are stepping up a presence at transport hubs and major airports, but that is purely for the purpose of reassurance and does not reflect any intelligence we have about a threat to London.”
Greater Manchester Police said they have also increased patrols.
Assistant Chief Constable John O’Hare said: “First and foremost, our thoughts are with all those who have been affected by the incidents in Brussels today.
“There is no specific threat in Greater Manchester following the events this morning but we remain at a threat level of severe.
“We are working closely with Manchester Airport to ensure that the appropriate response is in place and we have increased our patrols to support this.
“We will continue to review the situation and maintain our increased patrols in crowded areas, iconic locations and transport hubs.
“The events today in Brussels reinforce the need for us all to remain vigilant and report any concerns whilst continuing with normal daily life.”
Mr Rowley added: “In London specifically, the Metropolitan Police Service has mobilised additional officers who will carry out highly visible patrols at key locations around the capital, including the transport network.
“The number of officers deployed will be regularly assessed. These additional officers are deployed as part of reassurance measures.
“The police presence across London and the rest of the UK is constantly under review.”
He added: “We are in close liaison with the Belgium authorities and will continue to monitor the situation.
“The threat to the UK from international terrorism remains at severe as it has been since August 2014, meaning an attack is highly likely.”
He urged the public and businesses to be “alert but not alarmed” and report anything suspicious to the confidential Anti-Terrorist Hotline on 0800 789 321.
Brussels attacks: Nicola Sturgeon reacts
Scotland’s First Minister has expressed her horror at the Brussels blasts.
Speaking at a hate crime conference in Glasgow, Nicola Sturgeon said: “We are gathering to talk about tolerance, diversity and tackling hate crime as quite horrific news is emerging of what appears to be a devastating terrorist attack in Brussels.
“My thoughts are with everybody caught up in these events and indeed with the people of Belgium.
“It is a stark reminder of the importance of work we are doing to tackle hate crime and to strengthen diversity and community cohesion.”
Lord Advocate Frank Mulholland also expressed his sympathy.
Labour MEP Catherine Stihler said she was inside the Dutch Embassy in Belgium.
She tweeted: “My team and I are safe. I am still locked down in the Dutch Embassy.Thoughts and prayers with the 23 killed and their families this morning.”
Stephen Gethins MP, SNP Europe spokesperson, spent four years living and working in Brussels.
He said: “This is devastating news from Brussels this morning. My thoughts are with everyone who is affected by today’s atrocities.
Brussels is a wonderful city, multi-national and cultured with people living and working together from across the globe. It stands for the very best in what these terrorists hate most.
“I know that Bruxellois of all nationalities and backgrounds will be resilient in the face of these horrors.”
Flights today from Edinburgh Airport to Brussels have been cancelled and passengers are being advised to check with their airline for alternative travel.
Glasgow Airport tweeted: “In light of events at at Brussels Airport we are continuing to maintain our high levels of security. Our thoughts are with all those affected.”
Celtic player, Dedryck Boyata, who is a Belgian international, tweeted: “My thoughts and prayers are with the victims £zaventem.”