ANKARA (Reuters) - A prominent Turkish journalist was detained for trial on Friday, accused of participating in a coup by sending out subliminal messages to rogue troops who tried to seize power, media said.

Ahmet Altan, also a popular a novelist, was first held for questioning with his brother Mehmet Altan two weeks ago, both of them accused of sending out the messages during a TV talk show a day before the abortive July coup, state media reported.

Ahmet was freed on Thursday, but a prosecutor then argued he might flee and he was formally detained, pending trial, according to the P24 news website that he writes for. Mehmet never left custody.

Both will face trial "for trying to overthrow the government or prevent it from carrying out its duties," media outlets said.

Turkey has detained more than 100 journalists since July 15, when soldiers commandeered tanks and fighter jets, bombing parliament and other key buildings in an attempt to seize power.

The brothers, outspoken critics of President Tayyip Erdogan, allegedly disseminated "subliminal messages suggestive of a coup attempt" during their TV appearance on July 14, the state-run Anadolu Agency said at the time of their first detention.

Ahmet Altan, former editor-in-chief of the now-defunct Taraf newspaper, has denied the charges in a post on P24, calling them "nonsense".

The government blames followers of U.S.-based cleric Fethullah Gulen for masterminding the coup, but he denies involvement.

Some 100,000 police, soldiers, judges and civil servants have also been sacked or suspended on suspicion of links to Gulen, drawing criticism from rights groups and Western allies who fear a wider attempt to silence dissent. Tens of thousands of civil servants are in custody.

Turkish officials reject the criticism, saying the extent of the crackdown is justified by the gravity of the threat to the Turkish state on July 15. Those found not to have links to the coup plot will be released, officials have said.

Left-leaning Taraf, one of dozens of media outlets closed since the coup attempt, was once supportive of Erdogan's policies and his ruling AK Party. It has denied financial links with the Gulen movement.

More than 200 writers worldwide, including Margaret Atwood, Russell Banks and JM Coetzee, and other public figures have signed a petition protesting against the arrests of the Altan brothers.

(Writing by Ece Toksabay; Editing by Ayla Jean Yackley and Andrew Heavens)