Evan Gershkovich declares his innocence in a Moscow court, but his detention is upheld.

Evan Gershkovich, the Wall Street Journal reporter arrested in Russia last month, declared his innocence in a Moscow courtroom on Tuesday after flashing a smile from a glass defendant’s cage, his first public appearance since his detention on espionage accusations marked a new escalation in President Vladimir V. Putin’s conflict with the West.

The hearing was closed, but one of his lawyers, Maria Korchagina, told reporters that Mr. Gershkovich said he was to ready to “assert his right for free journalism” and “to defend himself.”

Earlier, journalists had been allowed into the courtroom where Mr. Gershkovich stood behind a pane of glass, with two masked officers in dark plainclothes to his right. Red handcuff marks were visible on his wrists. Dressed in jeans and a checkered shirt, Mr. Gershkovich blinked and nodded when one Russian reporter told him, “Evan, hold on!”

The judge, as expected, denied Mr. Gershkovich’s appeal to lift his pretrial detention, leaving him in Russian custody.

“Evan is wrongfully detained, and the charges of espionage against him are false,” the leaders of the Journal and Dow Jones, the paper’s publisher, said in a statement. “We demand his immediate release and are doing everything in our power to secure it.”

It was the first time that Mr. Gershkovich, a 31-year-old American, had been seen clearly since he was detained on March 29 while on a reporting trip in the Russian city of Yekaterinburg and accused of espionage, a charge the United States and press freedom groups also vehemently reject.

The case represents one of Mr. Putin’s most dramatic attacks to date on freedom of the press. It is the first time that the Russian government has brought such serious charges against a journalist officially accredited by the country’s own Foreign Ministry, and the first time a Western journalist in Russia has been charged with espionage since the Cold War. U.S. officials are concerned that the case appears to signal an even more severe Kremlin crackdown on independent news media outlets and the free flow of information within Russia.

Mr. Putin’s spokesman has claimed that Mr. Gershkovich was caught “red-handed” and signaled that the Russian president personally approved of the arrest. Russian authorities have not provided any evidence to support the accusations.

Outside the courtroom after the hearing, Mr. Gershkovich’s legal team said that the court had rejected an offer from Dow Jones to post a 50 million ruble — $600,000 — bond on Mr. Gershkovich’s behalf. Tatiana Nozhkina, a lawyer for Mr. Gershkovich along with Ms. Korchagina, said he was not guilty and later, in response to written questions, added that the legal team would appeal his arrest by filing a complaint about the lower courts’ decisions.

“He is in a fighting spirit,” Ms. Korchagina said. “He stated, accordingly, that he is ready to prove that he is innocent.”

The case against Mr. Gershkovich has brought relations between the United States and Russia to a new low. The Biden administration has asserted that he is “wrongfully detained” — which means that the U.S. government sees him as the equivalent of a political hostage held on fabricated charges — and called for his immediate release. If convicted, Mr. Gershkovich faces up to 20 years in a Russian penal colony.

Speaking from the steps of the Moscow City Court building where the hearing took place on Tuesday, the U.S. ambassador to Russia, Lynne M. Tracy, said that it was “troubling to see Evan, an innocent journalist, held in these circumstances.” She said that the “charges against Evan are baseless” and added that the U.S. government was “calling on the Russian Federation to release him.”

A day earlier, Ms. Tracy met with Mr. Gershkovich at Lefortovo Prison, his first consular visit since being detained. The visit followed repeated calls from the State Department for Russia to grant access to him.

Mr. Gershkovich was expected to be sent back to the prison after the hearing. It is one of the most infamous detention centers in Russia, where inmates are reportedly held in intense isolation with only rare visits by lawyers.

The case has prompted an outpouring of support for Mr. Gershkovich from colleagues, press freedom groups and international officials.

On Monday, Linda Thomas-Greenfield, the U.S. ambassador to the United Nations, condemned Mr. Gershkovich’s detention in a statement made on behalf of dozens of countries. “We urge Russian Federation authorities to release those they hold on political grounds, and to end the draconian crackdown on freedom of expression, including against members of the media,” she said.