International leaders united in sympathy and condemnation on Friday after a man ploughed a truck through a crowd into the front of a department store in central Stockholm.
Police said they had arrested one man after the attack, which killed four people and injured 15, while Swedish Prime Minister Stefan Lofven said "everything pointed" to it being a terror attack.
Foreign Minister Boris Johnson said on Twitter that he was "deeply concerned by shocking incident in Stockholm."
"Britain's thoughts are with the victims, their families and the whole of Sweden," he said.
European Commission President Jean-Claude Juncker said the attack was a blow struck against all EU countries.
"An attack on any of our member states is an attack on us all," Juncker said in a message of condolences to the victims, adding the aim appeared to strike at "our very way of life."
Antonio Tajani, the head of the European Parliament, said on Twitter that he was "shocked by the terrible news from Stockholm."
President Francois Hollande expressed his "horror and indignation" at the attack.
"France expresses its sympathy and solidarity with the families of the victims and all Swedes," he said.
Paris Mayor Anne Hidalgo announced on Twitter that the Eiffel Tower, which is normally illuminated, would go dark for a minute at midnight in honour of the victims of the attack.
Chancellor Angela Merkel's spokesman said Germany stood "together against terror" with Sweden, and offered sympathy for those involved in the attack.
The country's Foreign Ministry said on Twitter that "We strongly condemn the terrorist attack in Stockholm."
"Our thoughts are with the families of the victims and the people of Sweden."
The foreign ministry in Budapest condemned the attack in a statement to AFP, saying that "the Swedish people can count on Hungary in the fight against terrorism."
"It is unthinkable that innocent people are walking in a city centre street, and then suddenly they are the victim of a brutal attack. We condemn this appalling act," the statement said.
Dutch Prime Minister Mark Rutte called the attack "terrible news" and said he had conveyed his country’s condolences to his Swedish counterpart.
"Our thoughts go out to the victims and survivors," he said in a message on his Twitter account. "NL stands ready to help where needed."
"In our country, we are well familiar with the crimes of international terrorism. At this difficult time, Russians weep with the Swedish people," President Vladimir Putin said in a statement.
Prime Minister Mariano Rajoy said he was "closely following the attack in Stockholm."
"Condolences to the victims and best wishes for recovery to the wounded. We stand by your side Sweden," he said in a Twitter message.
UN Secretary General Antonio Guterres condemned the attack and voiced sympathy for the victims' families.
"We hope that those responsible for the attack will be swiftly brought to justice," he said.