"Three exceptional events are in the offing on 17 March, celebrations to mark the [152nd anniversary of the] unification of Italy, the probable enthronement of a new pontiff and the 19th Rome marathon," Alemanno stated.
''If the new pope is enthroned on that day, the marathon will be postponed until the afternoon," Alemanno told an event in Rome presenting the upcoming race.
Most cardinals have arrived in Rome this week for daily pre-conclave meetings but the Vatican has not yet set a date for the conclave - the election of a new pope.
In the clearest sign so far that a papal election is nearing, the Vatican on Tuesday announced the closure of the Sistine Chapel - the conclave venue - until further notice.
During the conclave 115 cardinal electors will hold secret votes several times a day to choose a new pope backed by a two-thirds majority.
The Holy See is currently vacant since 85-year-old Benedict abdicated on 28 February saying he lacked the physical and mental strength to remain in the job. Cardinals have said they would like to appoint his successor by the start of Easter week on 24 March.