Swedish Prime Minister Stefan Löfven held an impromptu press conference soon after the news broke.
Other Swedish leaders took to Twitter to express their sympathy and shock at the events in central Stockholm, with Anna Kinberg Batra, the leader of the largest opposition party, the Moderates, urging people to avoid speculation.
"A terrible attack in Stockholm", she writes, "my thoughts go to the families of the victims. It is important to avoid speculation and to let the police and others do their job."
International figures have also passed on their sympathies, the head of the European Commission Jean-Claude Juncker promises that the EU will do what it can to help the Swedish authorities. Belgian Prime Minister Charles Michel called this "a sad day", but added that he believed in the resilience of the Swedish people.
Meanwhile, Paris Mayor Anne Hildago said that the lights on the Eiffel Tower will be dimmed tonight as a tribute to the inhabitants of Stockholm.