Vadakkan, who joined the saffron party in the presence of Union minister Ravi Shankar Prasad and later met its president Amit Shah, said, "I am deeply hurt and that is why I am here."
Asserting that the Congress questioned the integrity of armed forces, he said, "The attack by Pakistan on our land and the reaction from my party was sad indeed."
"I left the party with heavy heart. If a party is working against national interest, there is no other option but to leave it," he told reporters.
Vadakkan told the media that he was deeply hurt at the way the Congress questioned the integrity of the armed forces after the suicide bombing at Pulwama led to India-Pakistan tensions.
In an apparent attack on party chief Rahul Gandhi's leadership of the Congress, he said he gave the party two decades of his life but it has now resorted to a practise of "use and throw".
Dynastic politics has reached its zenith in the Congress, he said, adding that he believed in Prime Minister Narendra Modi's development narrative.
The BJP may field Vadakkan in the Lok Sabha elections from a seat in Kerala.
Tom Vadakkan's departure non-issue: Congress leaders
Congress leaders in Kerala on Thursday said the departure of Congress spokesperson Tom Vadakkan is a non-issue for the party.
Congress spokesperson Joseph Vazhakken said that as far as the party in Kerala was concerned, Tom Vadakkan was never involved at any level in the state.
"We have only seen Vadakkan on TV channels. In Kerala he has had no space or role. His leaving is not going to affect us in any manner," Vazhakken told the media.
Congress Secretary and former MLA P.C. Vishnunath said he failed to understand the noise over Vadakkan joining the Bharatiya Janata Party (BJP).
"Vadakkan is one person who will not be able to make any impact even in a local body in Kerala. Who is he after all? Our only plea to the BJP is to see that they retain him forever. The Congress in the past has seen several similar opportunists and their going makes no impact at all," said Vishnunath.
Vadakkan, who hails from Thrissur in Kerala, had ahead of the 2014 Lok Sabha polls tried to use his influence in Delhi to get a Lok Sabha seat from Thrissur but Congress leaders and activists thwarted his attempt.
But Kerala BJP President P.S. Sreedharan Pillai said that many more Congress leaders were expected to join the party.
"Vadakkan is just one. You just wait and see, more are coming. But I will not reveal who they are," said Pillai.
Till now he used to 'abuse' PM Modi: Randeep Surjewala
After its spokesperson Tom Vadakkan joined the BJP, the Congress on Thursday dismissed his criticism of the party saying that till now he used to "abuse" Prime Minister Narendra Modi.
"Our best wishes to him," Congress's chief spokesperson Randeep Surjewala said when asked about Vadakkan joining the BJP.
Asked about Vadakkan citing dynastic politics and the Congress's stand on the Pulwama terror attack and India's retaliatory strikes as his reasons for quitting the party, Surjewala said every person who leaves a party to join another party "searching for a better future", has to say something.
"Till now he used to abuse Prime Minister Narendra Modi, so what is the opinion of Modi ji and Ravi Shankar ji on that," he said.
(With inputs from IANS)