The opposition's move to field Kumar makes it a Dalit vs Dalit presidential poll.
The opposition on Thursday unanimously decided to field Meira Kumar, a Dalit, as its joint candidate for the presidential election against National Democratic Alliance's Dalit activist Ram Nath Kovind, with 17 parties supporting the former Lok Sabha speaker's candidature.
The decision was taken at a meeting of the 17 non-NDA parties in Parliament House Library, with the leaders endorsing the name of the former minister and diplomat, whose father, ex-deputy prime minister Jagjivan Ram, was often hailed as one of the tallest dalit leaders in the country.
"We have 17 opposition leaders present here and all of us have decided to jointly field Meira Kumar as a candidate for the forthcoming election for the President of India," Congress President Sonia Gandhi announced after the meeting.
Asked to comment on Nitish Kumar's Janata Dal-United, which is supporting Kovind and stayed out of meeting, she said, "We do hope that other opposition parties join us." Gandhi added that she was "not upset with anybody".
Her party colleague, senior Congress leader Ghulam Nabi Azad, said Kumar had been "selected unanimously". All the 17 parties had proposed her name, he said.
He added Kumar was the "illustrious daughter" of former defence minister Jagjivan Ram.
"There could not be a better candidate than her," he said.
Communist Party of India-Marxist's Sitaram Yechury said "we are appealing to everybody" to support her.
After the meeting Lalu Prasad of the Rashtriya Janata Dal said he would meet Bihar chief minister Nitish Kumar and "appeal to him again to rethink" his party's decision to support the NDA nominee, which he called a "historic blunder".
He also said there was no threat to the Bihar government, a coalition of the JD-U, the RJD and the Congress.
Bahujan Samaj Party supremo Mayawati extended her party's support to Meira Kumar, saying she was "more capable and popular" than Kovind.
"The opposition's nominee for President's post Meira Kumar is more capable and popular than the NDA candidate," she said moments after the announcement of Kumar's candidature.
"The BSP will support Meira Kumar in the presidential election," she said.
The brief meeting of the opposition parties started with an address by Gandhi where she stressed the need for opposition unity in putting up a joint candidate.
She then gave the floor to Nationalist Congress Party leader Sharad Pawar, who said the names of three Dalit leaders -- Kumar, former minister Sushilkumar Shinde and former Maharashtra MP Bhalchandra Mungekar -- had been discussed for the top post.
When many in the group said Kumar was the best possible candidate, Gandhi proposed the name of the former speaker.
Earlier, Yechury had put forward the name of Prakash Ambedkar, but few among the leaders seconded him.
He also told the gathering that former diplomat-governor Gopalkrishna Gandhi, who was earlier being considered as a joint opposition candidate, had also suggested after the NDA named Kovind that the opposition put up a Dalit candidate.
The meeting was attended by a galaxy of leaders, including former prime minister Manmohan Singh, Congress leaders Azad, Mallikarjun Kharge and Ahmed Patel, NCP's Pawar, RJD's Prasad, Left leaders Yechury and D Raja, DMK's Kanimozhi and the National Conference's Omar Abdullah.
Though Trinamool Congress's Mamata Banerjee, BSP's Mayawati and SP's Akhilesh Yadav, who were present at the last opposition meeting held on May 26, were not present, their representatives attended the meet.
Derek O' Brien represented the Trinamool, Ramgopal Yadav, the SP and Misra, the BSP.
There were also representatives from the JD-S, RSP, JMM, Kerala Congress, IUML and the AIUDF of Assam.
Though the JD-U skipped the meeting, Rashtriya Lok Dal chief Ajit Singh was there, making up for the numbers.
Most of the leaders were present at the May 26 luncheon meet hosted by Gandhi at the same venue.
The Congress said it was "proud" that the party had fielded its second dalit candidate and the second woman for the post of the President.
Azad urged the leaders, especially those who had travelled from their states to Delhi, to sign Kumar's nomination papers as proposers or seconders before returning home.
Meira Kumar appealed to the electorate to take their decision in the best interest of the country based on cherished values of social justice and inclusiveness and on principles and ideologies.
She also thanked the 17 opposition parties for supporting her candidature and said she is delighted by the opposition unity that represents the coming together of forces which have a strong ideological base.
"I would appeal to the collegium to take their decision on the best interest of the country, based on the cherished values and principles and ideologies.
"These are social justice, inclusiveness and values of composite Indian heritage which we hold so sacred," she told reporters after she was declared as the unanimous opposition candidate.
Talking about the unity among opposition parties that came together to unanimously propose her name for the presidential election, she said, "this unity represents the coming together of the forces which have strong ideological base."
She said she was going to contest the election as the representative of a united opposition on the basis of the strong ideological base they have.
"I express gratitude to 17 opposition parties who selected me as a candidate for the presidential election," she said.
