'This is sad and scary': Videos show aftermath of Barcelona terror

Warning: This video contains graphic content

A graphic video of aftermath of the Barcelona terror attack has been posted online showing bloodied bodies strewn across the pavement.

The clip, shared on Twitter, shows the lifeless bodies of six people lying on the ground, some in pools of blood.

Horrified onlookers can be seen running for their lives as sirens blare in the background.

Paramedics are shown tending to the injured, including one who appears to be a young child.

Up to 16 people died and another 80 were injured when a white van ploughed into crowds at the busy tourist strip Las Ramblas in the heart of Barcelona, Spain, on Thursday afternoon (local time).

Local police say it was a terrorist attack.

"I was in Calle Hospital when I saw people coming from Las Ramblas running and shouting. I didn't know what was happening so I ran too," Nats Sisma Villaluna wrote alongside a video he posted on Facebook.

"This is sad and scary."

Witnesses have told CNN that the van appeared to deliberately "zigzag" along the pedestrian mall, presumably to inflict maximum damage to the people.

An unidentified witness told Spanish station TV3 there was "a hell of blood".

Police have so far arrested two men whom they are "treating as terrorists".