The consortium, which has headquarters in Switzerland, said members of the shareholders' committee met Monday near Portovaya Bay, Russia, to "mark the occasion of the flow of gas through Line 2."
Russian energy company Gazprom, which leads the consortium, said the last sections of the second string of the Nord Stream natural gas pipeline were connected at Portovaya Bay in late August.
Both lines, once fully operational, will transport about 1.9 trillion cubic feet of Russian gas each year to European consumers for at least 50 years.
The system is part of a network of pipelines planned by Gazprom to diversify Russian natural gas exports to European consumers. The first leg of the pipeline system went into service last year.
Nord Stream spokesman Jens Mueller said the start of Line 2 wouldn't translate for more gas for Europe, however. The project has been operating at about 35 percent of its total capacity during the first stages of operations, reports Russian news agency RT.
Nevertheless, the consortium said it was considering an extension of the project as outlined in a feasibility study.
"The feasibility study confirmed that the extension of Nord Stream with one or two additional lines is possible from the technical, environmental, permitting and financing perspectives," the consortium said.