The report by Association for Democratic Reforms and National Election Watch, which relied on the IT returns and list of donors submitted to the Election Commission for the period 2004-11, said Congress' income was Rs 2,008 crore while BJP earned Rs 994 crore.
Congress spokesman Manish Tewari said the party would comment only after going through the report, but a senior leader defended the donations, saying there was nothing "illegal or unethical."
Echoing same sentiments, BJP spokesman Shahnawaz Hussain said the party receives funds from its workers through a "transparent system of fund collection."
The NGOs said donations and voluntary contributions seem to be one of the major sources of income for most political parties and demanded more transparency in functioning of electoral trusts run by corporates added that political parties must be declared as public authorities.