Self-proclaimed anarchists burst into the Spanish embassy in Athens on Wednesday and threw leaflets in favour of Catalonian independence, an embassy official said.
The embassy staff evacuated the premises leaving only a few diplomats inside, the official said. There were no reports of damage or injuries.
"About 15 to 20 entered the embassy and threw leaflets. They did not break anything. The leaflet said 'solidarity is the weapon of the people'," the embassy official told Reuters.
The demonstrators unfurled a banner from the roof of the building reading "No Pasaran!" (They Shall Not Pass), a leftist slogan from the Spanish Civil War.
Greek police later detained 19 people, a police official said. Another police official said the group, known as Rouvikonas, had been demonstrating in favour of Catalonian independence.
"We are now back inside, the demonstrators have left," the embassy official said.
Small scale attacks against police, businesses, embassies and state buildings are frequent in Greece, which has a long history of political violence and is struggling to emerge from crisis despite three international bailouts.
Rouvikonas has stepped up its activities in recent months.