Amid the horror of the lorry attack in Stockholm, a photograph of an armed police officer patiently helping an old lady across the road as she limps with a crutch has captured the imagination of social media.

Taken in the aftermath of the attack, was posted by reporter Frida Sundkvist, it has already been shared hundreds of times.

One person who retweeted the post said it would have been the news picture of the year if it had been taken from the front instead of the back.

Another replied: "Wonderful image. Police and paramedics are making a great effort today."

The incident is being treated as a terror attack after the lorry was hijacked and driven at speed into the crowd near a department stall in central Stockholm.

Four people died in the attack and one person is understood to have been arrested.

Witnesses told Swedish daily paper Aftonbladet they saw hundreds of people fleeing for their lives.

"I went to the main street when a big truck came out of nowhere," one told the tabloid. "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.”