"A deal was struck between Kejriwal and Congress in a five-star hotel, a person (familiar with the development) told me," he said at a party function here.
Gadkari, who is party in charge of Delhi, alleged the Congress strategy was to prevent formation of a Bharatiya Janata Party government in the capital and stop Narendra Modi from becoming prime minister.
The Congress, which has eight legislators, is providing outside support to the Aam Aadmi Party, which won 28 seats in the 70-member Delhi assembly. The BJP has 31 seats. The remaining three seats are with the Shiromani Akali Dal, the Janata Dal-United and an Independent.
AAP formed a government after the BJP refused to do so, citing its lack of a simple majority.
BJP leader Arun Jaitley described the new Delhi government as a "strange situation" and charged Kejriwal with going back on his word.
"He said they will not take support from either Congress or BJP. It is a violation of public statement. AAP will have to introspect," he said, claiming the Congress supported AAP as it does not want a repoll in Delhi in which it fears its tally may further go down.
Hitting out at the AAP, Gadkari described the party's leaders as "liars" and "right wing Maoists".
"The BJP does clean politics, therefore (party's chief ministerial nominee) Harsh Vardhan refused to form a government. They (AAP) are liars. They are right wing Maoists. The people (of Delhi) will realize their true colours in the coming days," he said, accusing the AAP of being hand in glove with the Congress.
"The Congress and AAP are two sides of the same coin. We will fight both," he said.
"How will those who ditched Anna, relate to the people of Delhi," he said.
Leader of the Opposition in the Lok Sabha Sushma Swaraj urged party workers to enter into a "competition of honesty between the Delhi government (under AAP) and the three municipal corporations (under BJP)".
"Our councilors will now be watched closely. They must work selflessly among people. If you do this, the BJP will win all the seven seats in Delhi during the 2014 Lok Sabha poll," she told party workers.