Suhail Hindustani, a former Bharatiya Janata Party (BJP) worker, was arrested for the 2008 cash-for-votes controversy in parliament, making him the second accused to be taken into custody, police said on Wednesday.
"He was arrested after around eight hours of questioning," said a police officer, adding that Hindustani came to their office at 10.30am and was quizzed till 6pm.
Police Tuesday issued notice to Hindustani, asking him to appear for questioning before July 20.
Rajya Sabha member Amar Singh's former private secretary Sanjeev Saxena was the first person to be arrested in the case July 16.
Saxena had alleged that Amar Singh, who was then a member of the Samajwadi Party, had provided Rs 1 crore for buying the support of three BJP MPs in a trust vote in 2008.