Police began taking up positions and re-routing traffic ahead of a visit by Russian President Vladimir Putin which is expected to coincide with student protests Thursday.
Some 3,000 police will provide security in Athens when Putin arrives from Italy later Wednesday.
Putin will attend a signing ceremony Thursday for a pipeline agreement to transport Russian oil across Bulgaria to a port in northern Greece.
Thousands of students are expected to gather in central Athens shortly after the midday ceremony to demonstrate against education reforms that have kept universities closed for up to three months.
A similar student rally last week erupted in violence outside parliament, with clashes lasting more than three hours, 61 arrests and more than 20 injuries.
A repeat of the clashes on Thursday could embarrass Greek authorities who have been negotiating the on-off deal for more than a decade, and have presented the agreement as a major achievement for Prime Ministers Costas Karamanlis' conservative government, reports AP.
Karamanlis will attend Thursday's ceremony, together with Prime Minister Sergei Stanishev of Bulgaria.
The discovery of the submarine has unveiled a few "inconsistencies." For example, how can one explain the fact that the sub was found where it needed to be searched for from the start?
When on a state visit to Singapore, Russian President Vladimir Putin promised to revisit the discussion of the 1956 Declaration between the USSR and Japan regarding the issue of the peace treaty with Japan
The TurkStream, which runs along the bottom of the Black Sea from Russia's Anapa to Turkey, will consist of two lines, each with a capacity of 15.75 billion cubic meters of gas a year