The premier is a defendant in two trials in Milan accusing him of corruption and tax fraud. Both proceedings have been suspended thanks to the legislation, but will resume if the law is overturned by the Constitutional Court.

Berlusconi has always denied wrongdoing.

With journalists jammed in the courtroom, the 15 constitutional judges opened a hearing Tuesday during which the parties will lay out their cases.

A decision is not expected until Thursday.

The law, bitterly contested by the opposition, was passed in March by Berlusconi's conservative forces who control parliament, and went into effect the following month. It marked the third attempt at shielding Berlusconi during his various stints as head of government.

The legislation suspends court proceedings for up to 18 months if the defendant has a "legitimate impediment" stemming from being premier or a member of government. It was designed as a stopgap measure to buy the conservatives time while they prepare a more thorough immunity law for top officials.