Superstar footballers Cristiano Ronaldo, Luis Suarez, Neymar and Andres Iniesta have offered their condolences on twitter after the Barcelona terror attack on Thursday night.
There have been unconfirmed reports of gunshots in the area.
The thoroughfare is one of Barcelona's most famous landmarks, and is a hotspot for tourists from across the world.
Ambulances can be seen rushing past to help the victims.
The death toll could rise, with more than 100 injured, authorities said. In response to the police department's request, people began posting images of cats and other animals.
The Blaugrana condemned the deadly attack on Twitter, pledging solidarity with those affected.
Fellow Barcelona side Espanyol also registered their disbelief at events, writing "The most beautiful city in the world, we suffer for you and all Barcelona residents".