WASHINGTON – Lawmakers moved yesterday to create a 200-mile fence on the Arizona border as part of a larger effort to reduce the number of illegal immigrants.

Building a fence along part of the 2,000-mile southwestern border in an attempt to halt the flow of illegal immigrants from Mexico, now estimated at more than 500,000 a year, has emerged as a contentious potential solution to America’s border security problem.

The House passed a bill last year that would create a 700-foot fence along the most heavily trafficked parts of the border. Senators drafting an immigration bill, however, have been divided over the barrier, with a split among those who advocate for an actual fence, a “virtual” fence, or no fence at all.

The chairman of the Senate Judiciary Committee, Arlen Specter, a Republican of Pennsylvania, has pushed for a “virtual” fence using cameras and other technology to monitor illegal movement across the border. Some Republicans, such as Senator Cornyn of Texas, said even a virtual fence would send a “dramatically negative message” to America’s neighbors, while others said even a 200-mile fence does not go far enough.

The senators yesterday accepted an amendment from Senator Kyl, a Republican of Arizona, to create a combination of fencing, cameras, and barriers along part of his state’s border with Mexico.

Others proposed building double- or triple-layered “see-through” fencing in populated areas along the border in Arizona, and adding at least 200 miles of vehicle barriers to areas known to be transit points for illegal cross-border traffic.

Rather than constructing a fence along the whole length of the American-Mexican border, Mr. Kyl of Arizona, who introduced the amendment, said, “It’s important that we target our limited resources to the critical sectors of our country that will result in the greatest impact to stopping the flow of illegal crossings.”

He advocated that the Arizona fence should follow the example of the one around San Diego, where a triple-layered fence has greatly reduced the flow of illegal immigrants and drug smuggling into California. Just 10% of illegal immigrants apprehended on the southwestern border now come through the San Diego sector, down from 47% before the fence was built, in 1992.

However, the effectiveness of the San Diego fence has not reduced the overall flow of illegal immigrants to America from Mexico, but shifted the traffic to other parts of the border, in particular to Arizona.

“Nearly half of illegal aliens crossing the southern border of the United States enter through Arizona in the Tucson and Yuma Sectors,” Mr. Kyl said. The effects are felt as far away as New York. Since the stepped-up enforcement around San Diego, the majority of New York’s illegal immigrants are likely to have crossed the border into Arizona.

Debate also turned on making the act of entering America illegally, and of helping others to enter illegally, a criminal offense. The bill under consideration in the Senate, which is similar to the one that already passed in the House, would make unlawful entry a federal crime rather than a civil offense, as it is now. In addition, those who knowingly provide assistance to illegal immigrants could be charged with a federal crime and sentenced to prison.

Senator Durbin, a Republican of Illinois, introduced an amendment to strike down the measure, arguing that the Senate bill in its current form would make a criminal of an Oklahoma Republican, Senator Coburn, a physician, because he provides nonemergency medical care to illegal immigrants. Yesterday, the move was deferred and will be reconsidered when the committee reconvenes next week.

Critics have warned that making illegal immigration a criminal offense would set off a nationwide dragnet among the 12 million already living in the country illegally, which would be an impossible undertaking.

Cardinal Roger Mahony, leader of the Roman Catholic archdiocese of Los Angeles, the largest in America, last week called on church leaders to defy the law if enacted. Senator Clinton also warned that making criminals of illegal immigrants moved the country toward a “police state.” This week tens of thousands of immigrants across the country are protesting at the proposed changes in the law.

Mr. Durbin said introducing the criminal law into illegal immigration was the wrong starting point for a discussion on comprehensive immigration reform and was already frightening millions of Americans, as well as immigrants.

“Deportation is serious,” he said, “but it is not someone being charged with a crime tried in a court and possibly incarcerated for up to six months. That is a serious leap from where we are today.”