SAN FRANCISCO -- Mexican citizens formed long lines at their consulates in the United States on Monday, a day before a new federal law takes effect requiring passports for air travel between the United States and its neighbors.

Until now, travelers were able to use birth certificates, social security cards, drivers licenses or other documents to fly between the U.S. and Mexico, Canada and Bermuda. The new rule requiring passports for travel to and from those countries was recommended by the 9/11 Commission and adopted by Congress because passports are more difficult to forge than the other documents.

The requirement for air travel goes into effect on Tuesday; passports will be required for travelers entering the U.S. by land or sea as early as January 2008.

The line of people applying for passports at the Mexican consulate in San Francisco Monday morning curled around the block. Cruz Garcia, a Mexican citizen living in Hayward, had been waiting in line since 5 a.m.

"It seems important for the American government to know who comes and goes," she said. She plans to visit her parents in Mexico this summer and wants to be ready. "I don't want any glitches."

About 97 million people enter the United States by air every year. Although the majority hold passports, many use combinations of documents that are easier to forge. In 2005, the federal government intercepted 75,000 fraudulent documents, said Mike Milne, spokesman for U.S. Customs and Border Protection.

"It's about security -- border security," said Milne.

Canadian consulate officials in the U.S. were also seeing an increase in passport applications from "snowbirds," the approximately 70,000 Canadians who spend the winter here.

Mexican consulates are struggling to meet the demand for passport applications, which in some locations were three times higher than usual.

"We've increased our capacity as much as possible," said Cesar Romero, spokesman for Chicago's Consulate General of Mexico. In recent weeks, the office has been handing out up to 250 passports per day, instead of the usual 100. "It's incredible. We're doing the best we can."

In San Francisco, the Mexican consulate has extended its work hours to meet the demand. The office usually delivers about 50 passports a day -- but in recent weeks it has averaged 140.

"I just want to have everything in order and be ready to go," said Leonardo Vargas, of Menlo Park, who took Monday off from his landscaping job to get passports for his family in anticipation of a trip to Mexico's Michoacan province next month.

A pilot program is helping Mexicans schedule passport appointments over the phone, but the service hasn't yet reached California. Callers are jamming phone lines at consular offices.

"The message hasn't gotten out clearly about who needs the passport and who doesn't," said Romero. "It's generated anguish, worry, and left people nervous."

The only valid alternatives to the passport will be a NEXUS Air card, used by some American and Canadian frequent fliers; a document used by U.S. merchant mariners; and the "green card" carried by legal permanent residents. Active members of the U.S. Armed Forces are also excluded.

The new regulation brings Mexicans, Canadians and Bermudans into compliance with regulations for other foreigners entering the United States.

The next step, requiring travelers crossing borders or arriving in U.S. ports to hold valid passports, will affect many more people. Nearly three times as many travelers come into the United States by land or sea as by air.

"It makes sense: If the rest of the world needs a passport, Mexicans should too," said Romero. "But it won't be easy."