Pemex, the state-run energy company, said the new transportation methods had caused delays in the delivery of gasoline to service stations. The long lines, it said, did not result from actual fuel shortages.

“Pemex appeals to the general public’s support and understanding,” the company said in a statement. “These operations will undoubtedly translate into benefits for all Mexicans.”

Mr. López Obrador said Friday that 4,000 military and police personnel had been deployed to secure strategically important portions of pipeline, which stretch out some 375 miles and ordinarily transport around 400,000 barrels of gasoline per day.

He did not say when fuel would resume flowing through the closed sections, but vowed to continue with his efforts to stem the thefts. The thefts have been a result in part of the “incompetence or complacency” of local authorities, he said.

“That is what we are fighting against,” Mr. López Obrador said, “because corruption cannot be allowed.” He, too, said there was enough fuel in the country: “It is a matter of distribution only.”

Despite the long lines at the pumps, Mr. López Obrador showed no sign of changing his approach. He said the amount of stolen fuel had dropped significantly as a result of his tactics.