The closed-door trial of two Turkish journalists accused of espionage and aiding a terrorist organization resumed Friday in a case that has raised concerns over press freedoms in Turkey amid a crackdown from the government on dissenting speech.

Cumhuriyet newspaper's chief editor Can Dundar and Ankara representative Erdem Gul face life imprisonment if found guilty of revealing state secrets over 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 has been accused of aiding the moderate Islamic movement led by U.S.-based cleric Fethullah Gulen, a foe of President Recep Tayyip Erdogan.

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

Human rights groups insist the two have done nothing but their job by covering an issue of public interest and say the charges should be dropped.

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 case is considered a key test of press freedom in the country, which has witnessed a growing crackdown on independent and opposition media over the past few years.

"The ones who should be on trial are not us," Dundar said before the start of the second hearing.

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," Foreign Ministry spokeswoman Sawsan Chebli told The Associated Press. "That's why we expressed a desire to the Turkish side that a representative of the (German) consulate in Istanbul can observe the trial."

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 Turkish president is facing increased criticism for his government's crackdown on free speech at home. Speaking in Washington, D.C., on Thursday, Erdogan insisted no journalist is in prison or on trial in his country because of their journalism. He also said he welcomed criticism but would not tolerate insults.

"I would (thank) each and everyone one of those who criticize me but if they were to insult me, my lawyers will go and file a lawsuit," he said, according to Reuters.

The Associated Press contributed to this report.