MOSCOW, March 4 (Reuters) - Russia accused the United States on Wednesday of using an imaginary threat from Iran as a pretext to overthrow its constitutional order, and said Washington's calls for Iranians to seize power from their leaders were cynical and inhumane.Foreign Ministry spokeswoman Maria Zakharova said the U.S. had used negotiations with Iran, which were taking place as recently as last week, as a cover to disguise a plan for regime change there.