|Amar Singh, centre, has been sent to New Delhi's Tihar jail [EPA]
Indian police have arrested a high-profile member of parliament on charges that he bribed several lawmakers to support the government in a crucial confidence vote three years ago.
Tuesday's arrest of Amar Singh, in the so-called "cash-for-votes" case, brought immediate demands by opposition leaders for the Congress-led government to say who in its party was behind the alleged bribery.
"The beneficiary of the cash-for-vote scam was the government," said Rajiv Pratap Rudy, a leader of the main opposition Bharatiya Janata Party, demanding a deeper probe into the case.
"It is time for Amar Singh to say on whose behalf he acted."
Singh, 55, is accused of bribing three opposition MP's to back the government in the confidence vote that was sparked by heated opposition to a nuclear pact with the United States.
The politician from the northern state of Uttar Pradesh has denied the allegations.
He was sent to New Delhi's Tihar jail where two former government ministers, senior government officials and top corporate executives have been imprisoned on graft accusations.
The issue of corruption has rocketed up the political agenda in fast-developing India.
Anger over graft
Public outrage has mounted over corruption scandals - notably a telecom licence scam that allegedly cost the country up to $39bn in lost revenue and graft surrounding last year's Commonwealth Games.
At the time of the 2008 confidence vote, Singh was leader of a Socialist party allied to the government. He currently sits in the upper house of parliament.
At a hearing after his arrest, Singh requested bail on the grounds that he recently underwent a kidney transplant and would be at risk of infection in jail.
The request was denied.
A police chargesheet alleged that "sufficient evidence" had come to light that Singh "hatched a criminal conspiracy" to deliver cash to the lawmakers "as illegal gratification" for their votes.
The government narrowly survived the confidence vote.
Prime Minister Manmohan Singh, who is no relation to Amar Singh, has denied that any government or Congress party member was involved in buying votes.