A man has been arrested by police in connection with the attack
It's now believed that four people have been killed and 15 injured after a vehicle was driven into passers-by outside a department store on Drottninggatan.
According to police "One person has been arrested who may be connected with the incident."
Swedish police earlier released images of a man they want to talk to after a truck drove into pedestrians in Stockholm, killing three people and injuring many more.
Police have said there was no indication an attack was imminent, and no arrests have been made.
A suspect is thought to have run towards an underground station and a manhunt is underway.
Swedish Prime Minister Stefan Lofven has said everything indicates the incident is "a terror attack".
Mr Lofven earlier said one person had been arrested, but this has been contradicted by police.
The incident happened on one of the city's main shopping streets, Drottninggatan.
Local media reports the company who owns the truck, Brewery Spendrups, said was taken earlier in the day.
The company's communications director Marten Lyth, told Swedish paper Aftonbladet the vehicle was stolen as the driver was unloading.
He said the driver was unharmed but shocked.
The Irish embassy in Sweden says it is aware of "an ongoing incident in central Stockholm".
It is advising all citizens to remain in a safe place indoors.
Anyone with concerns can ring the embassy on 08-54-50-40-40 or ring Dublin on +353-1-408-2000.
The Embassy is aware of an ongoing incident in central Stockholm. We advise all citizens to remain in a safe place indoors @dfattravel— Irish Embassy Sweden (@IrishEmbSweden) April 7, 2017
Swedish broadcaster SVT says shots have been fired in the area, although security services are yet to confirm.
There are also reports of a separate shooting incident in a second location about 20 minutes from the pedestrian shopping street where the crash took place.
According to local reports, one eye witness in the shopping centre said she saw "hundreds of people" running for their lives.
Another witness described the panic, saying: "I went to the main street when a big truck came out of nowhere. I could not see if anyone was driving it but it got out of control. I saw at least two being run over. I ran as fast as I could from there."
Others reported hearing screams and seeing "a huge truck" crashing into the wall of a shop in the Åhléns shopping department at 1.00pm local time.
Thick smoke could be seen coming from the building, as police urged people to evacuate the area.
Armed police have cordoned off the scene, and police cars, helicopters and ambulances are at the scene.
The underground system has also been evacuated by police.
Local journalist Peter Wolodarski explains what seems to have happened.
"The lorry driver first drove into people on Drottninggatan - which is one of the main central streets in the city.
"A mainly pedestrian street and a main shopping area of central Stockholm. It then drove into the big department store, Åhléns" he said.
European Commission President Juncker said: "Our thoughts are with the people of Sweden following the tragic events in Stockholm today.
"On behalf of the European Commission, I would like to send my sincere condolences to the families of the victims and to salute the courageous work of the first responders who arrived so quickly on the scene.
"One of Europe's most vibrant and colourful cities appears to have been struck by those wishing it – and our very way of life – harm.
"We stand shoulder to shoulder in solidarity with the people of Sweden and the Swedish authorities can count on the European Commission to support them in any which way we can.
Minister for Foreign Affairs Charlie Flanagan has recently released a statement about the attack in Stockholm.
He said "I was shocked and saddened by reports of a serious incident this afternoon in Stockholm which has resulted in a number of fatalities. I wish to offer my sincere condolences to the families and loved ones of those killed and injured in this horrific incident.
"While the nature of the incident has not yet been officially confirmed, I understand that it is being treated as a terrorist attack by the Swedish authorities. I wish to express my own and the Government’s solidarity and support to the people of Stockholm and Sweden at this time.
"The Irish Embassy in Stockholm is monitoring the situation and at this early stage we have no information to indicate that any Irish citizens have been affected.
He finished by saying "Anyone concerned about family members in Stockholm should contact my Department on 01 408 2527 or the Embassy in Stockholm on 00 46 854 50 40 40”
Additional reporting: IRN