The 17-metre monument of Vladimir, the prince who converted modern Russia's precursor state Kievan Rus to Christianity in 988, has sparked controversy over fears it could blot Moscow's historic centre.

The opening of the statue -- backed by the influential Orthodox Church -- comes as the authorities under Putin continue to stoke a wave of nationalism fired up by Moscow's 2014 annexation of Crimea and subsequent standoff with the West.

"Prince Vladimir has gone down forever in history as the unifier and defender of Russian lands, as a visionary politician," Putin told officials as Russia marked its national Unity Day holiday.

"Today our duty is to stand up together against modern challenges and threats by basing ourselves on this spiritual legacy."

The decision to erect the monument just yards from the famed Kremlin walls has sparked an outcry from local conservationists and prompted concerns from UNESCO that it could jeopardise the Kremlin's status as a World Heritage site.

Local authorities only came up with the location after tens of thousands of Muscovites signed a petition against an earlier plan to place the monument on a prominent hill overlooking the city.

The statue was initially intended to be some 24 metres tall and weigh approximately 300 tonnes but it was scaled down after it was shifted to central Moscow.

The figure of Vladimir -- a warlord who killed his brother and, according to legend, had some 800 concubines -- has become increasingly politicised since Moscow seized Crimea from ex-Soviet neighbour Ukraine.

Vladimir was christened in the ancient city of Chersonesos on the Crimea peninsula, with Putin using the spiritual ties to the region to strengthen Russia's claim over the area.

But opponents of the statue say Vladimir had no historic links to Moscow, which did not exist during his lifetime, and point out that the kingdom he ruled was based around the modern Ukrainian capital Kiev.