Parliament passed the law on Thursday, making illegal immigration a criminal offense.  The bill has already been condemned by the center-left opposition and human rights groups including Amnesty International.

Monsignor Agostino Marchetto, a top official in the Holy See office concerned with migrants and refugees, said the law's approval marked a "dark day" for the country.

The legislation calls for jail terms for people who house illegal immigrants, and lengthens the amount of time migrants can spend in detention while authorities work to expel them.  It makes entering or staying in Italy without permission a crime punishable by a fine.

"It's a law that criminalizes and demonizes the many foreigners who come here asking only for help," Marchetto told La Repubblica, a left-leaning Italian daily. 

The bill also allows citizen patrols to help increase security in Italian cities and towns—a measure critics say paves the way for vigilante justice.

"Those who approved the law didn't put themselves in the shoes of thousands of illegals," Marchetto was quoted as saying by the moderate daily, La Stampa. "The law only worries about security and doesn't address the need for an environment that favors integration."

Premier Silvio Berlusconi's conservative government insists the measure will increase security, a key electoral promise. Many Italians link crime to illegal immigration, and the government has taken a tough stance against both. Speaking to Corriere della Sera, Italy's top daily, Marchetto predicted that the measures would only cause "more pain and difficulties" for migrants.

Foreign Minister Franco Frattini said that Italy "respects the Vatican's concerns," but insisted the measures "will not bring suffering, but only punishments for those who commit crimes".

Tens of thousands of migrants enter Italy by sea or alnd every year.  Some 36,000 migrants from Africa and elsewhere arrived in 2008 in rickety boats operated by smugglers out of Libya and other countries.

As part of its crackdown, Italy has also started sending back to Libya boatloads of migrants it intercepts in international waters without first screening them for asylum claims.  The move has been criticized by the UN Refugee Agency and rights groups.