Catherine the Great - Wikipedia

On June 28, 1762, Russian Tsarina Catherine II seizes power, declaring herself sovereign ruler of Russia. From the article:
"In July 1762, barely six months after becoming emperor, Peter took a holiday with his Holstein-born courtiers and relatives to Oranienbaum, leaving his wife in Saint Petersburg. On the night of 8 July (OS: 27 June 1762),[22] Catherine the Great was given the news that one of her co-conspirators had been arrested by her estranged husband, and that all they had been planning must take place at once. She left the palace and departed for the Ismailovsky regiment, where she delivered a speech asking the soldiers to protect her from her husband. Catherine then left with the regiment to go to the Semenovsky Barracks, where the clergy were waiting to ordain her as the sole occupant of the Russian throne. She had her husband arrested, and forced him to sign a document of abdication, leaving no one to dispute her accession to the throne.[23][24] On 17 July 1762—eight days after the coup and just six months after his accession to the throne—Peter III died at Ropsha, at the hands of Alexei Orlov (younger brother to Grigory Orlov, then a court favourite and a participant in the coup). Historians find no evidence for Catherine's complicity in the supposed assassination.[25]

At the time of Peter III's overthrow, other potential rival claimants to the throne existed: Ivan VI (1740–1764), in close confinement at Schlüsselburg, in Lake Ladoga, from the age of six months; and Yelizaveta Alekseyevna Tarakanova (1753–1775). Ivan VI was assassinated during an attempt to free him as part of a failed coup against Catherine: Catherine, like Empress Elizabeth before her, had given strict instructions that he was to be killed in the event of any such attempt. Ivan was thought to be insane because of his years of solitary confinement, so might have made a poor emperor, even as a figurehead.

Although Catherine did not descend from the Romanov Dynasty, she had descended from the Rurik Dynasty, which preceded the Romanovs. She succeeded her husband as empress regnant, following the precedent established when Catherine I succeeded her husband Peter the Great in 1725. Historians debate Catherine's technical status, whether as a regent or as a usurper, tolerable only during the minority of her son, Grand Duke Paul. In the 1770s, a group of nobles connected with Paul (Nikita Panin and others) considered a new coup to depose Catherine and transfer the crown to Paul, whose power they envisaged restricting in a kind of constitutional monarchy.[26] Nothing came of this, however, and Catherine reigned until her death."