In a major step to reform Colombia's battered judicial system, the government has created a special corps of "invisible judges" to hear drug- and terrorism-related cases.

Up to 500 judges will try cases without having their identities revealed, Justice Minister Jaime Giraldo told a news conference Tuesday. They will live in heavily protected fortresses in Colombia's four biggest cities.The reform measures Giraldo announced also provide anonymity for witnesses and have police, rather than judges, collect evidence in sensitive cases.

They are part of President Cesar Gaviria's program to protect judges from intimidation by drug traffickers.

Seventy-seven judges have been murdered by the cocaine bosses in the past 12 years. The magistrates often are given a choice between "plata o plomo" - a bribe or a bullet.

So far, the country has been unable to keep any major drug trafficker in jail. Most of the top traffickers brought to justice were first taken to the United States, and tried and jailed there under a special extradition treaty.

But the extraditions have created political problems because many consider them insults to national pride. Gaviria has said that fortifying the judicial system will allow the government to prosecute drug criminals in Colombia.

In the present system, judges must act as investigators and prosecutors in addition to deciding guilt or innocence. They visit crime sites and collect witness testimony, thus becoming even bigger targets of the drug mafia.