By Mustafa Yukselbaba

ISTANBUL, Oct 1 (Reuters) - Hundreds of demonstrators fearing a state coverup of the murder of a Turkish-Armenian editor demonstrated outside an Istanbul courthouse on Monday proclaiming: "We are all witnesses. We demand justice."

The EU, which opened membership talks with Turkey in 2005, sees the case of Hrant Dink as a litmus test for a judicial system often accused of conservative political bias.

Police imposed heavy security outside the court house where 19 suspects were being tried over the killing of Dink, gunned down outside his Istanbul office in January by a 17-year-old who has confessed to the killing.

"We are all witnesses, we demand justice," said banners held aloft by the protesters outside the court as the trial resumed in the Besiktas district of Istanbul.

Dink's lawyers have complained that the murder has not been properly investigated and have expressed fears for the independence of the court, reflecting concerns about the possible involvement of Turkey's so-called "deep state".

The "deep state" is a term coined to describe hardline nationalists in the bureaucracy and security forces who are prepared to subvert the law for their own political ends.

At the weekend, Turkey's liberal Radikal newspaper published the transcript of a conversation between one of the suspects and a police officer two hours after the shooting which it said showed the officer was aware of a plan to kill Dink.

The Interior Ministry has launched a probe into the the telephone conversation.

Dink had angered Turkish nationalists with his comments on the massacres of Armenians in Ottoman Turkey during World War One. More than 100,000 people turned out at his funeral to show solidarity and protest against violent nationalism. Lawyers were expected to question the suspects for the first time at Monday's hearing. Eight suspects are in custody.

Media reports have said one of the suspects had repeatedly tipped off police about a plot to kill Dink and that these tip-offs had been conveyed to the Istanbul police headquarters.

Several officials, including the head of police intelligence in Istanbul, have been sacked or reassigned to other jobs over their handling of the Dink case.

Ankara denies Armenian claims, backed by many historians and by a growing number of foreign parliaments, that the killings amounted to a systematic genocide. It says large numbers of both Muslim Turks and Christian Armenians died in ethnic fighting as the Ottoman Empire collapsed during World War One.

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