London mayor Sadiq Khan said the city “stands united with Stockholm” after a hijacked truck ploughed into a shopping centre in the Swedish capital killing four people and injuring 15 more.
Mr Khan said it appeared Sweden had seen “a despicable act of terrorism aimed at harming innocent people and attacking our shared values of democracy, freedom, justice and tolerance."
He said: “Londoners know how it feels to suffer from senseless and cowardly terrorism.
"And I know we share a steely determination with the people of Stockholm that we will never allow terrorists to succeed.
“We will never be cowed by terrorism and today London stands united with Stockholm.”
My thoughts and deepest sympathies are with the victims and families who’ve been affected by this horrific attack in Sweden. pic.twitter.com/FHeenbwQj8— Sadiq Khan (@SadiqKhan) April 7, 2017
The lorry drove into several people on Drottninggatan, a busy street in Swedish capital, before hitting the Ahlens shopping centre on Friday afternoon, police said.
Prime Minister Stefan Lofven said the incident which began at about 3pm local time appeared to have been "a terror attack".
He told reporters during a visit in western Sweden: "Sweden has been attacked. Everything points to the fact that this is a terrorist attack." He was immediately returning to the capital.
Widespread condemnation poured in from across the globe as news of the attack broke, including from German Chancellor Angela Merkel and Foreign Secretary Boris Johnson, who said he was "deeply concerned".
Mr Johnson said: “Deeply concerned by shocking incident in Stockholm. Britain’s thoughts are with the victims, their families and the whole of Sweden.”
The German Chancellor's spokesman said: "Our thoughts go out to the people in Stockholm, to the injured, their relatives, rescuers and police.
"We stand together against terror.”
President of the EU Commission Jean-Claude Juncker said: "An attack on any of our Member States is an attack on us all."
The foreign ministers of Germany and France reacted with shock to the deadly truck attack in Stockholm.
In a joint statement, Sigmar Gabriel and Jean-Marc Ayrault said Sweden's efforts for human rights, peace and justice around the world made the pictures emerging from “the heart of Stockholm that much greater.”
Separately, German President Frank-Walter Steinmeier expressed solidarity with the victims in Stockholm "and all people in Sweden."
Paris' mayor Anne Hidalgo said the lights on the Eiffel Tower would go out from midnight in solidarity with Stockholm.
She expressed solidarity with the victims and their loved ones, as well as with Stockholm Mayor Karin Wanngard "in this particularly difficult ordeal."
Russian President Vladimir Putin sent a telegram of condolence to Sweden's monarch in the wake of the fatal truck crash.
"In our country, people know, and not by hearsay, about the atrocities of international terrorism. At this difficult time, Russians mourn together with the people of Sweden," Putin said.
A suicide bomb on a St. Petersburg subway train on Monday killed 13 passengers.
The incident in Sweden comes after trucks were used in terror attacks in Nice and Berlin last year and just two weeks after Muslim convert Khalid Masood ploughed his car into pedestrians on Westminster Bridge in London.