Missouri governor Eric Greitens has been charged after allegedly taking a compromising photo of a woman with whom he had an affair in 2015.
The Republican governor responded that he made a mistake but committed no crime after allegedly taking a photo of an unnamed woman "in a state of full or partial nudity".
St. Louis Circuit Attorney Kim Gardner launched an investigation in January after Mr Greitens admitted to an affair with his St. Louis hairdresser that began in March 2015. He was elected governor in November 2016.
Thursday's indictment was followed with an announcement by House Republican leaders that they were forming a group of politicians to investigate the charges "and answer the question as to whether or not the governor can lead our state while a felony case moves forward".
In a statement following the indictment, the Republican governor was defiant and attacked the prosecutor who brought the charge.
"As I have said before, I made a personal mistake before I was governor," he said.
"I did not commit a crime. With today's disappointing and misguided political decision, my confidence in our prosecutorial system is shaken, but not broken.
"I know this will be righted soon. The people of Missouri deserve better than a reckless liberal prosecutor who uses her office to score political points."
Mr Greitens' lawyer, in a separate statement, called the indictment "baseless and unfounded."
"In 40 years of public and private practice, I have never seen anything like this," attorney Edward L. Dowd Jr. said.
Mr Greitens' legal team immediately filed a motion to dismiss the indictment on the grounds that any relationship with the woman was consensual.
Some politicians renewed suggestions that Mr Greitens should consider resigning, as they had done when the affair first become public last month.
Democratic state senator Jamilah Nasheed of St. Louis called for an impeachment process to begin immediately.
"Governor Greitens has to go," Mr Nasheed said. "Missourians thought they voted for a person of character and integrity, and instead they got a liar and alleged criminal."
Any impeachment process must begin in the House with an investigation.
The indictment states that on March 21, 2015, Greitens photographed a woman identified only by her initials "in a state of full or partial nudity" without her knowledge or consent.
The indictment said Mr Greitens "transmitted the image contained in the photograph in a manner that allowed access to that image via a computer."
In 2015, the woman told her husband, who was secretly taping the conversation, that Mr Greitens took the compromising photo of her at his home and threatened to use it as blackmail if she spoke about the affair.
The penalty for first-degree invasion of privacy in Missouri is up to four years in prison.
Mr Greitens was taken into custody in St. Louis and later released, and is due in court for his first hearing on March 16, before Circuit Judge Rex Burlison.
The politician has repeatedly denied blackmailing the woman, but has repeatedly refused to answer questions about whether he took a photo.