Citigroup Inc Chief Executive Vikram Pandit resigned abruptly on Tuesday (October 16), effective immediately, shocking Wall Street.

Pandit resigned from Citigroup, the No. 3 U.S. bank, just one day after a surprisingly strong quarterly earnings report.

Analysts and investors quickly raised red flags about the timing, saying it did not appear to be a natural transition and rather suggested some sort of dispute at the bank.

But Citigroup shares rose 1.5 percent in morning trading as some investors said they were not sorry to see the one-time hedge fund executive leave.

A statement from Chairman Michael O'Neill said Michael Corbat, previously chief executive for Europe, Middle East and Africa, would succeed Pandit as CEO and as a board member.

Chief Operating Officer John Havens, a long-time associate of Pandit, also resigned.

Pandit's successor, Corbat, has held a number of senior roles at Citigroup, including running Citi Holdings, the unit established to house businesses and assets the company wants to shed.