In a photograph shown by public broadcaster RTVE, three people were lying on the ground in the street of the northern Spanish city this afternoon, apparently being helped by police and others.
Videos of the scene recorded people screaming as they fled.
Police cordoned off the broad, popular street, ordering stores and nearby Metro and train stations to close.
They asked people to stay away from the area so as not to get in the way of emergency services. A helicopter hovered over the scene.