Russian President Vladimir Putin helped inaugurate India's biggest nuclear reactor unit via videolink on Wednesday.
The reactor, with a capacity of 1,000 megawatts, is the first at Kudankulam Nuclear Power Plant in the southern state of Tamil Nadu and has been active since early last year.
It was built using Russian technology, and about 80 per cent of its construction was financed with Russian loans.
A second reactor with the same capacity is scheduled to go into operation at the power plant by the end of this year. In total six reactors are planned for the plant.
India's Prime Minister Narendra Modi said the inauguration was a historic step in Indian-Russian relations and that it was just the beginning of Russia's cooperation with India on an ambitious nuclear energy programme.
Currently, only about 2 per cent of India's power generation capacity of about 300 gigawatts comes from nuclear power. The government wants to increase this to up to 25 per cent by 2050.