MOSCOW – President Putin unexpectedly sided with Russia’s Greens yesterday, ordering the re-routing of a big oil pipeline away from the world’s largest freshwater source.

Days before the start of construction of the 2,500-mile pipeline, he stipulated that it should run no closer than 25 miles from Lake Baikal instead of the planned 875 yards.

“If there is even a tiny chance of Baikal becoming polluted, then we, thinking about generations to come, must do everything not just to minimalize but to exclude it,” he told energy officials in Tomsk.

Russian scientists and environmentalists had opposed the original plans because of seismic activity that might breach the pipe and pollute Baikal through its tributaries. But the state company, Transneft, deemed alternate routes for the $11 billion pipeline to the Pacific too expensive because it would mean skirting mountains.

The turnaround came after protests last weekend in several Russian cities.

Greenpeace, which ran a campaign to protect the giant lake, welcomed the re-routing order as “magnificent and ecologically correct.”

But Mr. Putin’s intervention was reminiscent of previous occasions when he appeared to overturn or temper debated decisions at the last moment, in an attempt to increase his popularity.