The Russian Foreign Ministry has presented a special note to the Estonian Ambassador to Russia, where it stressed its opposition to plans by Tallinn to remove remains of Soviet soldiers.
The document says the reburial “will have the most serious consequences for relations between Russia and Estonia”.
Meanwhile, hundreds of Estonians have laid flowers at the site despite a police ban on a rally planned by the public movement “Night Watch”.
The activists wanted to persuade the authorities not to block off the area on Victory Day.
Work to move the remains of Soviet soldiers is due to begin in Tallinn on Wednesday.