The banners went up just as the Parthenon temple reopened to tourists
Protesters have hung huge banners on the Acropolis in Athens calling for "resistance" after days of street clashes between youths and police.
"Resistance" was written in four languages on one banner at the ancient site that dominates the Greek capital.
Another banner called for Europe-wide mass demonstrations on Thursday, when a nationwide strike is planned in Greece.
The riots across Greece were sparked by the fatal shooting of a teenager by a policeman on 6 December.
The Acropolis reopened to the public on Tuesday when Greek culture ministry staff ended a strike over pay that had lasted for 10 days.
The protests spread to national broadcasters on Tuesday, when 10 youths briefly seized a state NET TV studio, holding banners saying "Stop watching, get out onto the streets". They disrupted the broadcast of a speech by Conservative Prime Minister Costas Karamanlis.
Separately, youths attacked the riot police headquarters in the capital, Athens, damaging several vehicles.
Some 70 people have been injured in the wave of riots, in which hundreds of shops and banks have been vandalised and looted.
It is estimated that 400 people have been detained during the protests.
The policeman accused of shooting Alexandros Grigoropoulos, aged 15, has been charged with murder.
Mr Karamanlis has rejected calls to step down.
But on Tuesday he acknowledged some of the problems that had fuelled the anger of young people.
In a speech to parliamentary colleagues, he said "long-unresolved problems, such as the lack of meritocracy, corruption in everyday life and a sense of social injustice disappoint young people".