Armed police are currently at the scene and the incident is being treated as a suspected terror attack, according to local media.
The lorry had been hijacked earlier in the day.
The public has been warned to stay away from the city centre and a police cordon has been put up around the area.
Swedish Prime Minister Stefan Lofven has said everything indicates that the Stockholm truck crash is “a terror attack” and confirmed that one suspect has been detained.
He added: “The Government is naturally being updated continuously and receiving the latest information.
“The Government is assisting the authorities in its work and now we are encouraging the public to be alert and listen to the police’s information.”
Police have now confirmed a shooter at a second incident in Fridhemsplan – around a 20 minute drive from the first incident.
Shops on the street have closed down and people are being locked inside, according to reports.
One woman wrote on Twitter: “Friend are trapped at the dentist at fridhemsplan. Police say it is anxiously outside.”