NEW DELHI — The leaders of India’s ruling Congress party addressed a rare gathering of hundreds of thousands of supporters on Sunday, attempting to reclaim lost political ground after being battered by a series of corruption scandals.

The rally comes as a two key Indian states head to the polls and as the party begins preparations for national elections to be held in 2014.

Sonia Gandhi, the Italian-born leader of the Congress party, defended a series of politically unpopular economic reforms launched by the government during the last few months. These included opening India’s retail sector to foreign investment and cutting fuel subsidies.

In her fiery speech, Gandhi also addressed the corruption allegations her party and its allies have faced during the last two years.

‘‘I admit, corruption is a cancer, it is a disease,’’ she said. ‘‘We will continue to fight this disease.’’

Several government ministers are facing corruption charges stemming from scandals over the hosting of the 2010 Commonwealth Games, the sale of cellphone spectrum, and allocation of coal fields that auditors said lost the country billions of dollars.