"It is an injustice being meted out to Amar Singh. It is a conspiracy to trouble him," Mr Yadav told a press conference in New Delhi.
Amar Singh was questioned for three hours by the Delhi Police's Crime Branch on Friday over his alleged involvement in the infamous 2008 cash-for-votes scandal. Mr Singh is accused by the BJP of buying MPs to save the first UPA government.
The police claim they have got evidence that links Amar Singh to one of the two men arrested for trying to bribe three BJP MPs. They are likely to call the leader again next week.
Mr Singh refused to speak to the media, but sources say he stood his ground before the police, insisting he was not involved in the scandal.
When asked about the ten-odd bank accounts from which the bribe amount was allegedly withdrawn, Mr Singh said he did not know the account holders.
When he was shown phone records that Sanjeev Saxena, an accused in the case and also his aide, had called him six times on his official landline, Mr Singh said he employed many people and can't recollect Mr Saxena specifically.
It is the first time Mr Singh has given a statement in the cash-for-votes case. He had not deposed before the Parliamentary Committee that had probed the matter earlier.
The BJP MPs at the heart of the scandal are Ashok Argal, Faggan Singh Kulaste and Mahavir Bhagora. At the time of the no-confidence vote, Mr Singh's Samajwadi Party was a crucial ally of the UPA coalition at the Centre - it provided the PM external support.
The two men arrested by the police in connection with the scam last week are Sanjeev Saxena and Sohail Hindustani. Mr Saxena is allegedly caught on hidden camera offering the money to the BJP MPs, who say he introduced himself to them as Mr Singh's secretary. Mr Singh had denied that Mr Saxena was ever his aide, but the police have documents including a note from Mr Singh intended to help Mr Saxena's son get admission in a college. The letter clearly introduces Mr Saxena as his secretary.
The police have also extensively questioned Sohail Hindustani, who was arrested on Wednesday. Mr Hindustani allegedly tried to shop the BJP MPs around to Congress leaders before the trust vote. Several turned down his officer, according to the police. They claim that Mr Hindustani then approached the Samajwadi Party via its leader Rewati Raman Singh. The police believe that Amar Singh did not depute Mr Hindustani to find BJP MPs who could be bought. Rather, Mr Hindustani knocked on the doors of the Samajwadi Party, promising that he could deliver three BJP MPs.
Mr Hindustani's lawyers, who describe him as "a whistle-blower", accused the police of "giving a clean chit to the Congress and the Samajwadi Party" even before the investigations got into full swing. The BJP concurred.
The BJP's own role has also been questioned. A report in the Tehelka newsmagazine earlier this year revealed that the party may have urged its MPs to put themselves on the market ahead of the trust vote, in the hope of exposing the UPA's willingness to buy support. This is why, some say, the BJP also co-opted a TV channel to shoot the negotiations between the MPs, Mr Saxena and Mr Hindustani.
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