By Marty Graham

SAN DIEGO (Reuters) - Mexican demonstrators blocked or slowed traffic near at least four busy entry points along the United States's southern border on Monday to protest against new Mexican fees and rules on imports of used cars from the United States.

The Veterans International Bridge between Brownsville, Texas and Matamoros, Mexico remained closed on Monday evening, U.S. Customs and Border Protection spokesman Roger Maier said.

Traffic was clearing around 7:30 p.m. at the Otay Mesa entry point in San Diego, where protesters started blocking the Mexican approach in the morning, Otay customs officials said.

As late as 8 p.m., the cargo lanes at the port of entry between Calexico and Mexicali also remained closed, customs officials confirmed.

The U.S. State Department on Sunday warned U.S. citizens to be ready to encounter significant delays and problems at a number of U.S. Mexico border crossings.

"The protests concern changes in Mexican customs regulations for the importation of vehicles," it said in a message.

Mexico imported more than 640,000 used cars from the U.S. for resale in 2013, while about 1 million Mexican-made cars were sold, a figure that has stayed static for four or five years, a Mexican automotive industry association said in a statement.

Images on social media websites showed protesters on the Mexican side of the Otay Mesa port of entry in San Diego, and at other bridges and gates, holding signs condemning the new measures.

(Editing by Curtis Skinner and Clarence Fernandez)