The men were charged with "supporting terrorism" on Tuesday for writing a joint letter to Anders Fogh Rasmussen, the Danish prime minister ,last December.
The mayors had asked Fogh Rasmussen to ignore the Turkish government's call to close down Roj TV, a Denmark-based Kurdish television station.
Turkey says Roj TV is a mouthpiece of the Kurdistan Workers Party (PKK), an armed separatist group, which Ankara, together with the European Union and the US, classifies as a terrorist organisation.
Mayors defend free speech
The Turkish prosecution has said that by seeking to prevent the closure of the station, the mayors have committed the offence of "knowingly and willingly supporting" a terrorist group.
It says that Roj TV hosts PKK leaders, carries PKK statements inciting violence and follows a broadcasting policy "in line with PKK propaganda".
Among the accused is Osman Baydemir, one of Turkey's most popular Kurdish politicians and mayor of Diyarbakir, the largest city of the predominantly Kurdish southeast.
Osman Baydemir has great
popular support among Kurds
The mayors rejected the accusations, arguing that the letter aimed to defend freedom of press and the interests of the Kurdish people in the southeast, where Roj TV enjoys a wide audience.
One of the mayors, Firat Anli, said as he presented a joint defence on behalf of all defendants: "We stand behind each and every word of the letter... This trial is yet another reflection of the undemocratic approaches to the Kurdish question in Turkey.
"Rather than defending a certain channel, our letter underlined the need to tolerate different voices, even if they are dissident, and to ensure freedom of press and freedom of expression."
Turkey's EU ambitions
The mayors had written that silencing Roj TV "would mean the loss of an important vehicle in the struggle for democracy and human rights" in Turkey.
They said efforts to press Denmark into banning the channel contradicted Turkey's commitment to improve its democracy record in its bid to join the EU.
Fogh Rasmussen rejected Turkey's demands to close Roj TV and said that Turkey should not put the men on trial.
"I find it rather shocking ... that because you write a letter to me, you are being accused of violating the law," Fogh Rasmussen told Danish public radio in June.
"It is shocking that it can take place in a country which is seeking EU membership."
Recently, Turkey has come under intense scrutiny for putting a series of high-profile authors on trial.