It is written in Greek letters:

PROKLAMATSIA
NA ELLINOMAKENTOSKI KOMITET
OT ATINA
ZA NASITE BRATE MAKENTONΤSI

The language is the Macedonian language, translated into English it says:

PROCLAMATION
OF THE GREEK MACEDONIAN COMMITTEE
FROM ATHENS
TO OUR BROTHERS THE MACEDONIANS

Please notice the picture of Alexander the Macedonian.