Putin warned the European nations that Moscow has repeatedly said it would have to retaliate if the missile defense shield was put into place, but US allies have ignored Russia's warnings.
On Saturday, Putin will visit the Monastery of St. Panteleimon, which is inhabited by Russian monks.
After a marathon closed-door meeting between the two officials, talks continued between the two delegations, focusing on strengthening bilateral economic and trade relations, upgrading political dialogue on worldwide and regional developments and EU-Russia, NATO-Russia relations and cooperation in culture, education, science and technology.
During his visit to Mount Athos, where women are not allowed to visit any of the 20 monasteries there, Putin repeatedly praised the spirit of the monastic community.
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Some 2,500 police were providing security for Putin's visit in Athens, and much of the city center was blocked to motorists and public transport.