The 1980-batch IPS officer yesterday submitted his resignation letter to the Maharashtra Home Ministry, which forwarded it to the Chief Minister's Office for acceptance.
The top cop confirmed he has resigned from the Indian Police Service, but was yet to decide which political party to join.
"I have resigned and sent the papers to the State Government," Singh said today.
"I will work for national unity, social harmony and world peace. I have not yet decided which party to join. You will come to know about it in two to three days."
Singh, who was due to retire next year, is reportedly considering contesting the upcoming Lok Sabha polls either from Mumbai or Uttar Pradesh, his native state.
According to some media reports, he has offers from BJP as well as the Aam Aadmi Party.
The 59-year-old senior IPS officer, who has several books to his credit, was appointed Mumbai Police Commissioner, one of the most high profile posts in the Indian police establishment, in August 2012.
During a career spanning over three decades, Singh occupied several top positions in Maharashtra Police.
He served as Police Commissioner of Pune and Nagpur and was Additional Director General when he was chosen for the present assignment nearly one-and-a-half years ago.