In Moscow Russian President Vladimir Putin joined a column of people carrying portraits of their relatives who fought in the Great Patriotic War of 1941-45.
"According to our estimates, about 12 million participated in the march [Russia-wide]," Zemtsov told RIA Novosti.
The Moscow City Police reported that about 500,000 people participated in the event in the Russian capital only.
Earlier in the day, Moscow hosted a large military parade to commemorate the 70th anniversary of victory over the Nazi Germany in the Second World War.
The event was the biggest Russian military parade ever held, with about 16,500 troops and almost 200 units of military hardware participating.
The Soviet Union is estimated to have lost about 27 million people in World War II.
The military casualties exceeded 8.7 million, which is more than a half of the total allied death toll.
