A bomb blast early Monday damaged a building in Athens housing the headquarters of Greece's private radio and television network Skai, but there were no casualties, police said.
Anti-terrorist police opened an investigation into the attack that focused on Greek extremist groups.
Attacks targeting broadcasting groups, public companies or embassies have been frequent in Greece in recent years, and have been blamed on anarchist or far-left groups.
The coalition government led by leftist Prime Minister Alexis Tsipras condemned the blast as "an attack on democracy" while his administration decried an act of "terrorism."
In a statement, Tsipras slammed what he termed "an attack on democracy by cowardly and dark forces," vowing that "they will not realize their goal of terrorizing and disorientating."
He further offered his "sincere support to journalists and all those who work at the station" targeted.
The homemade device went off at around 2:30am (0030 GMT), 45 minutes after an anonymous telephone warning to another TV network.
Police cordoned off the neighborhood in the Athens suburb of Neo Phaliro and evacuated the building, which contains the offices of Skai, a group owned by the Alafouzos shipping family, as well as those of Kathimerini, a center-right daily critical of the government.
Police said the bomb was placed in a narrow street near a fence around the building and smashed windows on the facade.
Skai said in a statement the blast caused "major damage."
"The terrorist attack will not discourage us," it said, accusing the government of failing to do enough to protect the media despite "recurrent threats against the station."
Government spokesman Dimitris Tzanakopoulos rejected the accusation.
The minister for civil protection, Olga Gerovassili, visited the site with police.
"Democracy is not threatened," she said, while warning against those who "leave the way open to terrorism or fascism."
There were no claims of responsibility by late afternoon but some analysts said the bombing bore the hallmarks of an attack by the far left Popular Fighters Group (OLA).
It has previously claimed to be behind at least five other similar blasts, none causing fatalities, since its formation in 2013.
The group last claimed a bombing outside the Athens Court of Appeal in December 2017, which caused extensive material damage.
Last month, police defused a bomb outside the Athens home of a controversial prosecutor following two anonymous telephoned warnings to the media.