Congress chief Mallikarjun Kharge, general secretary Priyanka Gandhi Vadra, senior leaders K C Venugopal, P Chidambaram and Salman Khurshid were among the party's top brass taking part in the satyagraha at Rajghat.
Singh met Congress president Rahul Gandhi before joining the party.
The political drama in Rajasthan over leadership change continued on Monday with MLAs loyal to Chief Minister Ashok Gehlot remaining non-committal on holding a meeting with Congress observers, who are likely to return to Delhi to apprise the high command about the developments.
Congress president Rahul Gandhi met Rajasthan Chief Minister Ashok Gehlot and his deputy Sachin Pilot separately.
The prime minister also called for more RT-PCR tests.
Gehlot said even within the party, there is an opinion in favour of making Rahul the new president.
Party chief Rahul Gandhi formed a nine-member Core Group Committee, a 19-member Manifesto Committee and a committee comprising 19 top party leaders who will look after the publicity during the elections.
The party last saw a contest for the post in November 2000 when Jitendra Prasada had lost to Sonia Gandhi.
The 'Make Rahul Gandhi All India Congress Committee president again' chorus is growing louder in the Congress with half a dozen state units so far seeking his elevation to the top post, even as uncertainty and suspense persisted over whether he would take on the mantle.
Rajasthan Chief Minister Ashok Gehlot's son Vaibhav will contest from Jodhpur and Manvendra from Barmer.
In veiled criticism of the Ashok Gehlot government on Tuesday, Congress leader Sachin Pilot called for zero tolerance to incidents like the recent killing of a Dalit boy and called for action against local officials for an alleged lathi-charge on the victim's family.
No senior leader in the party wanted to react to Ashok Gehlot's comment that he would stop the culture of girls and boys going hand in hand to pubs and malls. The party spokesman Shakeel Ahmed when asked said that he did not know in what context the statement had been made and he would first have to find out.
In an apparent reference to the return of Sachin Pilot and other rebel MLAs, the chief minister said that he told the MLAs on Tuesday night that they have to bear and work in the interest of people.
Giriraj Singh Malinga said he had alerted Gehlot ahead of the Rajya Sabha polls that a conspiracy was going on to topple his government.
Ramesh Meena, who was among those BSP legislators who made the switch to Congress, said: "The chief minister should honestly tell how much money was paid to us when we joined Congress and left the the BSP in the last Congress government."
The meeting was called for discussing the points which were raised by Governor Kalraj Mishra on Monday.
Ahead of the launch of its 'Bharat Jodo Yatra' on Wednesday, the Congress said it is a turning point in Indian politics and marks a 'new beginning'.
He hoped that some of the dissident MLAs, who are led by sacked deputy chief minister Sachin Pilot, will also attend the session when it is called "very soon".
In a day of frantic developments, Digvijaya Singh entered the fray for the Congress president's election as Rajasthan Chief Minister Ashok Gehlot opted out of the race and the suspense over his continuation at the helm in the state deepened with the party saying it would be decided in a day or two.
"If anybody with a mass base joins the BJP or any political party, everybody welcomes him. Expressing trust in our ideology, if anybody joins us we will welcome him with open arms. This is a normal procedure," Union Minister Gajendra Singh Shekhawat told reporters in New Delhi.
Gehlot's comment comes after he came under fire from the Bharatiya Janata Party and other Opposition parties over the incident in which a temple priest was burnt alive in Karauli district over a land dispute.