Athens Centre at Archimidous 48 in Mets has taught modern Greek to generations of new Athens residents.  Their teachers have formidable intellectual qualifications, and the cultural enrichment is superb.  Their telephone is (210) 701-5242, web site is www.athenscentre.gr
Per Judy Blish, the Greek Ministry of Education adult education program has 80 sites throughout Greece offering lessons in basic through advanced Greek and computers for immigrant (workers, wives etc.) all for free. She went for 200 hours and had a wonderful time, great class of many nations and a terrific teacher. Courses also include English, creative writing in Greek, communication skills, citizens' rights, music appreciation, photography & cinematography. The website (in Greek) is www.ideke.edu.gr/kee Telephone of the central HQ is 210 258 4803-4.

In Athens, the Adult Education Center (KEE) is at Stratigou Dagkli 53, in Kato Patission, telephone 210 854 1078.  Classes are supposed to run twice a year, September - January and February - June.

The Citizen Service Center (KEP) for immigrants, located at the corner of Psarron and Mezonos (three blocks from Metaxourgeio metro station), also advertises Greek lessons for foreigners.