(CNN) -- At least two people were killed when a truck was driven into pedestrians on the busiest street in the center of Stockholm, in what appeared to be the latest use of a vehicle as a weapon of terror in Europe.
The Swedish Security Service said a large number of people were injured in the attack, which happened just before 3 p.m. local time on Friday. People were seen fleeing the area in panic.
Local media reported that the truck was hijacked as it made a delivery nearby. Eyewitnesses reported it being driven at speed along Drottninggatan (Queen Street) before it crashed into the front of a department store.
Security services said the driver of the truck was not in custody and a manhunt was underway. Police urged people to stay at home and avoid the city center.
Parliament and the Stockholm subway were placed in lockdown. All train services in and out of Stockholm Central Station were halted and people were evacuated, the Swedish state railway company said.
"Sweden has been attacked," Swedish Prime Minister Stefan Lofven told reporters "Everything indicates this is a terror attack."
Lofven said the government was doing everything possible to help the security services. "We are thinking about the victims -- their families and friends -- and those who were injured," he said."
The Prime Minister was returning to the capital from the west of the country, his spokesman Erik Nises told CNN.
King Carl XVI Gustaf also announced he had cut short his state visit to Brazil to return home immediately in the aftermath of the attack.
Anders Thornberg, Director General of the Swedish Security Service, said a "person of interest" was being investigated by police and released a photo of him to the public.
Police also confirmed they were talking to two individuals about the attack but stressed they were not suspects.
'He put his foot on the gas'
According to eyewitness Veronica Durango, the driver "put his foot on the gas and ran through the crowd."
"He came from Olof Palmes Street and drove down to Drottninggatan," Durango told CNN in a phone interview.
"It was like he was driving through paper. It's like it was nothing. I can't even believe how a person could do such a thing. And then he just kept on going. I was in shock."
Spendrups Brewery, which owns the truck used in the attack, said the vehicle was "hijacked" by a "masked man" shortly before 3 p.m. local time.
Spokeswoman Rose-Marie Hertzman told CNN that the company's driver attempted to chase after the truck but was nearly run over.
Security services launched an investigation into whether more than one people was involved in carrying out the attack.
"Plenty of people have been injured and some have died in the attack that just hit Stockholm," said Nina Odermalm Schei, head of press for the security service.
"We are still trying to determine who the attacker was, if the attack was carried by one or more people, and the number of injured."
Carl Bildt, the former prime minister of Sweden, wrote on Twitter: "Steal a lorry or a car and then drive it into a crowd. That seems to be the latest terrorist method. Berlin. London. Now Stockholm."
Jens Stoltenberg, the Secretary General of NATO, said: "My heartfelt sympathies go to all those affected. We stand with the Swedish people in their grief."
The Swedish Security Service first received notification at 2.55 p.m. on Friday that a truck had been driven into a crowd on Drottninggatan in Stockholm, a press officer told CNN.
Intensive intelligence work was underway in order to identify the person or persons behind the attack, the spokesman said.
The US embassy in Stockholm told US citizens to avoid the area of the attack.
The same area was hit by a failed attack in 2010. A suicide bomber died and two people were injured when his car bomb went off prematurely as he made his way towards the Drottninggatan street area, which was packed with Christmas shoppers.
The attack joins a growing pattern of vehicles being used to launch attacks on pedestrians.
Last month, a man rammed into crowds on Westminster Bridge in London, killing three people before stabbing a policeman to death outside Parliament.
A fifth victim, Andreea Cristea, who was knocked off the bridge and into the River Thames below, died after her life-support was switched off on Thursday afternoon.
In July 2016, more than 80 people were killed and over 200 injured when a terrorist used a 20-ton truck to plough into crowds who had gathered in Nice to watch Bastille Day fireworks.
Last December, 12 people were killed and at least 4