The explosives squad was called to intervene, at about 5 a.m., and that`s when the officer was gravely injured, police spokesman Cesare Taraschi said.
The blast was under investigation.
Italy occasionally sees explosions that authorities have blamed on political extremists. The country suffered bombings, slayings and kidnappings during a period of domestic terrorism in the 1970s and early 1980s.
Mayor Dario Nardella says peaceful Florence "isn`t represented by the horrors" of the blast.
Florence Cardinal Giuseppe Betori decried criminals who wanted a "bloody passage from the old year to the new."
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