BERLIN (Reuters) - German police placed concrete barriers in front of Cologne's world famous gothic cathedral on Wednesday after reports that Islamist militants sought to target Barcelona's La Sagrada Familia, another icon of ecclesiastical architecture.

Many German cities have tightened security since last week's van attack that killed 13 people in Barcelona, after a period in which most were wary of erecting barriers for fear of triggering a fortress mentality that would put off visitors.

"We took the decision to act as quickly as possible after looking at attacks in Europe," a police spokeswoman said. "Our job is to protect sensitive points, and the cathedral is a symbol of Cologne, known around the world."

Spanish media reported that the Islamist militant cell that sent a van plowing into strollers on Barcelona's showpiece Las Ramblas boulevard had been plotting an attack on Antoni Gaudi's art nouveau landmark, the Sagrada Familia church.

Cologne's immense cathedral, the work of unidentified medieval masons, towers over the Rhine river city. A UNESCO World Heritage site, the cathedral was one of the few buildings in Cologne's center to survive World War Two bombings.

Although there have been around a dozen attacks around Europe by militants driving trucks, cities have not rushed to mitigate risks by changing their layout. They have been deterred by high costs, the belief that any measures now will soon be obsolete and a reluctance to disrupt everyday life.

"We don't want to wall up the city," Andreas Geisel, Berlin's senator for interior affairs, said in an interview with Bild newspaper. "That would achieve the opposite of what we want: to send out an image of calm and relaxedness."

In the German capital, barriers remain in place around the site of a Christmas market where a militant drove a stolen truck into crowds and killed 12 people last year, but few other visible measures have been implemented.

(Reporting by Thomas Escritt; editing by Mark Heinrich)