NEW DELHI (Reuters) - Italian-born Sonia Gandhi took over as the new president of India's Congress party on Saturday but her appointment by the party elite was challenged by the man she replaces as illegal. The row over the change at the top in India's oldest political party came as its rival Hindu nationalist Bharatiya Janata Party (BJP) moved closer to winning enough support to form a coalition government in the aftermath of recent inconclusive elections. Sitaram Kesri said last week that he was ready to step down as party president in favour of the 51-year-old widow of slain former prime minister Rajiv Gandhi, but declared on Saturday that he was still in charge and had not resigned. But party leaders said Sonia was in control. Congress vice-president Jitendra Prasada told a news conference. Firecrackers went off at the single-storeyed colonial bungalow housing the party office as hundreds of supporters chanted slogans cheering Sonia, as she is called in her adopted country. Congress will hold its meeting of the All India Congress Committee (AICC), the general body which elects the CWC, in the first week of April. The CWC said earlier it had named Sonia because of Kesri's announcement that he would resign. Kesri's move last week created confusion in the ranks of the party as it struggled to find allies in efforts to stop the Hindu nationalists from taking power after winning the most seats in a hung parliament returned by a mid-term election. The BJP on Saturday gained support from a recalcitrant pre-poll ally to bolster its chances of being invited to lead the next government while Congress appointed its new leader. The Congress, which has ruled India for all but five years since independence from Britain in 1947, suffered its worst electoral defeat in 1996, and made no significant improvement this year, winning 167 seats with allies in the 545-member parliament. It has been presided over for most of these years by the Nehru-Gandhi family which has given India three prime ministers, Jawharlal Nehru, his daughter Indira Gandhi and her son Rajiv. Sonia entered the political fray last year after seven years out of public life following the assassination of her husband. She became the party's star campaigner in the last elections, often appearing on platforms alongside her daughter Priyanka and son Rahul. Kesri said he had wanted to summon a session of the AICC in front of which he would resign. "I am still the Congress president. This takeover is illegal," he said. "I had said I would respectfully step down at the AICC session and invited Sonia to take over respectfully but not in this way." Sonia met Kesri after she took charge but neither divulged details of the meeting.