Walking into Mexico at the nation's busiest border crossing with the United States is no longer an uninterrupted stroll for foreigners.

Starting late Wednesday, pedestrians going to Tijuana from San Diego at the San Ysidro crossing must choose between a line for Mexicans who get waved through, and a line for foreigners. Foreigners must show a passport, fill out a form and - if staying more than a week - pay roughly $20 for a six-month permit.

Travelers have long followed similar protocol at Mexican airports, but the change marks a big change at land crossings. Pedestrians and motorists have generally entered Mexico unencumbered along the border with the United States.

Mexican immigration Rodulfo Figueroa says the change is "about putting our house in order."