The European Union has added its voice to the abhorrence being expressed after a bomb attack caused extensive damage to the headquarters of a Greek media group that has been critical of the country’s left-wing government.

Skai is one of Greece’s biggest TV stations, part of a media outlet that also includes a radio station and the daily, Kathimerini.

Police said a makeshift explosive device went off near the Skai building at 2.37am local time (0030 GMT) following warning calls to two media companies. The calls provided enough time for no one to be injured, police said, describing the early Monday bomb blast as “especially powerful”.

The force of the blast was such that windows as high as the sixth floor were blown out of the glass-fronted building housing the two organisations. Anti-terror police who spent the day combing through the debris, concluded the explosion had been triggered by up to 10 kilograms of dynamite packed into a rucksack.

Greek police experts search for evidence outside the SKAI TV building. Photograph: Angelos Tzortzinis/AFP/Getty Images

There was no immediate claim of responsibility.

The EU’s economics chief, Pierre Moscovici, called the strike “an attack against our way of life, our principles and our values.”

European Commission spokesman Margaritis Schinas said it “condemns all attacks on the freedom of expression and our free press cornerstones of our values.”

He added: “These freedoms are fundamental to the continent, our union and our very democracies. I hope that this will be the last time that we have to state the obvious.”

The bag is believed to have been placed on a roadside barrier along the coastal boulevard that the media group looks onto in southern Athens.

“There were warning calls to [other media outlets] 45 minutes earlier which allowed the building to be evacuated,” police spokesman Theodoros Chronopoulos told the Guardian. “But the blast was especially powerful. It is too early to say who was behind it even if we have our suspicions.”

The blast bore similarities to an attack against the Athens appeals court a year ago that was also powerful, he said. The far-left Group of Popular Fighters subsequently claimed credit for targeting the leading symbol of the justice system.

Earlier in the day the Greek prime minister, Alexis Tsipras, also deplored the attack as the work of “cowardly and dark forces against democracy itself.”

Damage inside the offices of SKAI TV after the explosion. Photograph: STRINGER/EPA

“I express my frank solidarity with the broadcaster’s journalists and employees,” he said in a statement.

Tsipras’ leftist-dominated coalition has frequently clashed with the media group amid accusations of anti-government bias.

A boycott by cabinet ministers Skai TV and radio channels was launched this year because of coverage perceived to be prejudiced.

In a statement Skai said it had warned authorities it had been receiving threats but the notifications were not taken seriously. “Government officials and propaganda mechanisms have rendered our station a target,” it said. “Following today’s dramatic development they must understand the weight and full extent of their incendiary comments.”

Journalists, who responded by broadcasting outside the glass-shattered building, vowed they would not be gagged or intimidated by the attack.

On Tuesday the court of appeal in Athens was evacuated following a bomb scare.

Greece has a long history of political violence attributed to its turbulent past under military dictatorship before the return of democracy in 1974.

As the debt-stricken country has navigated its worst economic crisis in modern history – applying excoriating austerity in return for three international bailouts to stay afloat – attacks have increased again in recent years.