The 66-year-old lawmaker was formerly a minister in the western state of Maharashtra and was known for his lavish parties.
Siddique was shot several times in the chest outside his son's office, local media reported.
It comes weeks out from state elections slated to be held in November.
Investigation underway
Maharashtra's deputy chief minister, Ajit Pawar, who is from the same party as Siddique, said he was "shocked" by the "cowardly attack."
"The incident will be thoroughly investigated and strict action will be taken against the attackers," Pawar said in a statement.
"The mastermind behind the attack will also be traced."
The Hindustan Times newspaper reported that two suspects had been arrested, with police looking for a third. Broadcaster NDTV said the suspects claimed they were part of a gang run by Lawrence Bishnoi, who is in jail on charges of running a criminal gang that has carried out multiple killings.
