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Omer Kuscu, Associated Press
Turkish journalists cover their mouths with black ribbons before the trial of Can Dundar, the editor-in-chief of opposition newspaper Cumhuriyet and Erdem Gul, the paper's Ankara representative, outside the courthouse in Istanbul, Friday, April 1, 2016. The trial of two Turkish journalists accused of revealing state secrets and helping a terror organization over their reports on alleged government-arms smuggling to Syrian rebels was resumed on Friday after opposition lawmakers in the first hearing refused to leave the courthouse in defiance of a ruling that the case should be behind closed doors.

ISTANBUL — The closed-door trial of two Turkish journalists accused of espionage and aiding a terrorist organization resumed Friday amid tight security, in a case that has heightened concerns over press freedoms in Turkey.

Cumhuriyet newspaper's chief editor Can Dundar and Ankara representative Erdem Gul face life imprisonment if found guilty of revealing state secrets in their report on alleged government arms-smuggling to Syrian rebels.

They published images that reportedly date back to January 2014, when local authorities searched Syria-bound trucks, leading to a standoff with Turkish intelligence officials. Cumhuriyet said the images proved Turkey was smuggling arms to Islamist rebels.

The pair have been accused of aiding the moderate Islamic movement led by U.S.-based cleric Fethullah Gulen, a foe of President Recep Tayyip Erdogan. They deny that accusation.

The government says the reports are part of a conspiracy to bring it down.

Human rights groups insist the two have simply done their jobs by covering an issue of public interest and say the charges should be dropped.

The case is considered a key test of press freedom in the country, which has seen a growing crackdown on independent and opposition media in recent years.

"The ones who should be on trial are not us," Dundar said before the start of the hearing. The first session was held a week ago.

Dozens of supporters at the Istanbul courthouse where the two are on trial chanted: "Free press cannot be silenced." Others came with their mouths taped over in an act of protest.

The trial has also drawn the attention of Western diplomats, with Germany putting in a request for one of its diplomats to attend the hearings.

"This interest can be explained by the great importance that the German government accords to an independent judiciary and a free press," German Foreign Ministry spokeswoman Sawsan Chebli told The Associated Press.

The journalists were arrested in November after Turkish President Recep Tayyip Erdogan filed a personal complaint against the two.

In February, Turkey's Constitutional Court ruled that their rights were violated and they were released from jail. Erdogan said he rejected the court's decision and has also admonished diplomats who showed up at the first hearing.

The journalists were relieved that they were not re-arrested on Friday. Dundar said the hearing took place in "extraordinary conditions" with high security measures including three barriers to reach the courtroom. Gul said they had been questioned by the lawyers of the president and the national intelligence agency in addition to the prosecutor.

The Turkish president is facing increased criticism for his government's crackdown on free speech at home. Speaking at the Brookings Institution in Washington, D.C., on Thursday, Erdogan said no journalist is in prison or on trial in his country because of their journalism and that he welcomes criticism.

Berza Simsek and Bram Janssen contributed from Istanbul.