Illegal immigrants who land in Italy consistently allege they have been subjected to abuses, according to a report by Amnesty International.
Ilegal migrants can be detained for up to 60 days at special centres
Holding centres are often overcrowded and no legal assistance is offered to asylum seekers, the rights group says.
Amnesty says it could not investigate the allegations further as the Italian authorities refused to give it access to the centres.
It is calling for the creation of a monitoring body to check the centres.
"People are held in overcrowded, unhygienic conditions, have no contact with the outside world and have no access to legal advice," Amnesty's Nerys Lee, who conducted the research, told reporters in Rome.
According to allegations from migrants, they have been beaten, given sedatives and gagged by law enforcement officers and supervisory staff while being expelled from the country by plane.
"The number, consistency and regularity of the allegations... has led Amnesty to believe that there is a substantial cause for concern," Ms Lee said.
The human rights group says Italy is increasingly resorting to locking up asylum seekers in detention centres "in violation of international refugee standards".
Many are denied access to lawyers and experts, thereby making it impossible to challenge their detention or deportation order, it says.
Amnesty also raised concerns over the detention of irregular migrants who have not applied for refugee status.
The BBC News website contacted the Italian government, which was not immediately available for comment.
Tens of thousands of immigrants land on the Italian coast each year, most of them heading from north Africa on ramshackle boats.
Holding centres, like the one on the small island of Lampedusa, halfway between Libya and Sicily, are constantly overcrowded.
Amnesty wants Italy to set up a special watchdog that could carry out unannounced and unrestricted checks at the centres and other facilities designed for illegal migrants.
The report, which was released to coincide with World Refugee Day, says 15,647 people were held in the centres in 2004 - a 9% increase on the previous year.