The trial is largely seen by Kurdish activists as a test of Ankara's democratic credentials at a time when it is engaged in a cautious plan to end a deadly 26-year Kurdish insurgency.

The 7,500-page indictment accuses the suspects of involvement in the Kurdistan Associations Union (KCK), which prosecutors describe as a terrorist group that acts as the urban extension of the armed Kurdistan Workers' Party (PKK).

It calls for jail terms ranging from five years to life in jail on various charges including "leadership and membership of a terrorist organisation", "undermining the state's unity", "spreading terrorist propaganda" and "aiding an abetting a terrorist organisation".

Among the suspects is Osman Baydemir, the popular mayor of Diyarbakir, the regional capital of the mainly Kurdish southeast where the trial is being held.

Eleven other regional mayors from Turkey's main Kurdish party -- the Peace and Democracy Party (BDP) -- and former lawmaker Hatip Dicle are among the defendants.

Heavy security was in place outside the courthouse for the start of the trial which is also being followed by intellectuals and rights activists from Europe.

Defence lawyers and Kurdish activists have slammed the trial as a move to "silence Kurds".

"Our clients are being tried as unarmed members of an armed group," lawyer Meral Danis Bestas told reporters last week.

"This is a political trial aimed at silencing Kurds... This trial will serve as a lithmus test for Turkey's democracy and how it views the Kurdish conflict," she said.

The PKK, listed as a terrorist group by Turkey and much of the international community, took up arms for Kurdish self-rule in the southeast in 1984, sparking a conflict that has claimed some 45,000 lives.

Since August last year, the Turkish government has been engaged in a cautious two-pronged strategy of keeping the PKK under military pressure and expanding the rights of its sizeable Kurdish population in the hope of persuading the rebels to lay down their weapons.