"As it was warned time & time again that 'a bully will get bully-er if you succumb', the American bully now bites its EU allies," Mousavi wrote on his Twitter account on Monday.
"The US habit of aggressive law-breaking will only lead to global chaos. When will the world grasp the reality & rise up against this bully?" he added.
Nord Stream 2 is a 1200km-long offshore natural gas pipeline being constructed to connect Europe to the world's largest reserves in Northern Russia.
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