Prime Minister Vladimir Putin on Friday inaugurated a 25-kilometre-long dam aimed at protecting Russia's second city of Saint Petersburg from the threat of floods.

The Saint Petersburg Flood Prevention Facility stretches across the Gulf of Finland in one of Russia's most spectacular engineering projects of recent years.

It will ensure that the glittering and canal-strewn former imperial capital is spared being overwhelmed by high floodwaters, a risk it has lived with since its foundation by Peter the Great in 1703.

The construction of the dam started in 1979 but was suspended in the 1990s due to a lack of funds but was then revived in 2006. The total investment is 109 billion rubles (3.7 billion dollars).

"Without exaggeration, today is an historic event," said Putin as he inaugurated the facility. "It is worthy of Saint Petersburg, it's a grandiose facility. It has a technical beauty."

Putin said the dam would also improve the environmental situation in the city as the top of the dam would support part of the Saint Petersburg ring road and relieve traffic congestion in the city.