He also suggested abuses may have taken place after protesters were in custody.
The admissions are likely to fuel accusations of police brutality that have poured in from Europe and elsewhere, embarrassing the new government of Premier Silvio Berlusconi and raising questions about how future summits can be controlled.
In his first appearance before a parliamentary fact-finding committee, Chief Gianni De Gennaro told lawmakers that efforts before the summit to weed out violent anarchists and prevent them from reaching the Genoa meeting were "below expectations."
Once the summit began, he said, "the warlike conditions created by violent criminals led in some cases to an excess in the use of force from some units."
Police were unable to quell riots during the July 20-22 G-8 meeting, which saw the first death of an anti-globalization protester, more than 200 injured, 240 people detained and millions of dollars in damage. Dozens of protesters remain in jail pending hearings, including at least four Americans and 16 members of an Austrian theater group police say were conspiring to destroy the city.
De Gennaro said an internal investigation was under way, and he promised to take disciplinary action against any officer found to have taken part in "unlawful behavior" during the meeting. "There will be no reticence," he said.
On Wednesday, the Interior Ministry completed its investigation into the violence and recommended nine officers be disciplined for two of the most controversial episodes at Genoa: the pre-dawn police raid at a school housing protesters, and the treatment of demonstrators detained at the Bolzaneto police garrison.
Protesters in Italy as well as Britain, France, Germany and elsewhere have claimed they were beaten up by police, strip-searched and denied food, phone calls or access to their consulates after their arrests.
De Gennaro said he wanted to wait for all the investigations including at least eight separate ones by Genoa prosecutors to finish before he makes a final assessment of what went wrong.
But he said he couldn't rule out the possibility that some police were responsible for "illicit behavior" at the Bolzaneto police garrison. Protesters claim they were mistreated and in some cases beaten while in police custody.
The Interior Ministry in its report said it couldn't confirm charges that protesters suffered physical and psychological violence at the garrison. But, "one cannot rule out the occurrence of sporadic acts of roughness facilitated by a situation of extreme confusion," it said.
The Interior Ministry report also criticized the pre-dawn raid of the school housing protesters, saying it was poorly organized even though senior law enforcement officials were on the scene.
Protesters say hey were brutally beaten up by police, some clubbed in their sleep during the blitz. Over 90 people were arrested and 61 taken to hospitals. Hours after the raid, the school's floor and radiators were still stained with blood, windows shattered and computers and closets smashed.
An elite Italian police unit which carried out that bloody raid a was trained by U.S. police chiefs, an Italian newspaper reported on Tuesday.
For four months, 70 specially selected officers were trained by two Los Angeles police sheriffs. A larger number of police also received a week-long training course from the Americans, according to the Communist daily Liberazione.
Three top law enforcement officials, including De Gennaro's deputy and the superintendent of Genoa police, were removed from their posts by the government last week.
On Wednesday, the police union called for De Gennaro himself to resign, saying his acknowledgment of police excesses showed he too had failed.
The violence at the summit has prompted Berlusconi to shy away from holding a November U.N. food conference in Rome and Naples' mayor to ask that a September NATO meeting be moved.
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