ATHENS — A powerful bomb detonated outside a major Greek broadcaster and newspaper publisher near Athens early Monday, causing serious damage to the facade of the building but no injuries. Greek politicians across the spectrum condemned it as an “attack on democracy.”

The blast occurred at 2:37 a.m. outside the building of the Skai television channel and radio station, which also houses the newspaper Kathimerini, part of the same media group. A police spokesman said the explosion followed anonymous calls to another television station and news website shortly before 2 a.m.

“They said a bomb would go off in 45 minutes and kept repeating ‘this is not a hoax,’ ” said the spokesman, Theodoros Chronopoulos. “It was a big explosion.”

He said the bomb had been in a backpack that was hung on a barrier along the busy coastal road that runs in front of the Skai offices in suburb south of Athens. Greek media reported that it contained up to 10 kilograms, or 22 pounds, of explosives.