The case is being held in private at the Constitutional Court
Turkey's ruling party has been defending itself in court against charges that could see it closed down.
Prosecutors allege that the AK Party wants to impose Sharia law and is a threat to Turkey's secular status - claims dismissed by the party itself.
Prosecutors also want Prime Minister Recep Tayyip Erdogan and President Abdullah Gul barred from politics.
The case occurs amid high political tensions, after the arrest of several top figures over an alleged coup plot.
The AKP won a landslide victory in the last election a year ago, taking 47% of the vote.
It says the trial is ideological, not legal - launched by the traditional secular elite that failed to defeat it at the ballot box.
As Turkey's deputy prime minister led the defence in a closed hearing in the Ankara courthouse, AKP politician Suat Kiniklioglu told BBC News that the case should never have been brought.
"This is not about secularism versus conservative politics. This is really about what direction this country should be going," he said.
"Should this country become a normal, transparent, European-style democracy - this is exactly what we are trying do to - or should it remain a country that is state-dominated, where a small, exclusive elite runs the country as it has done for the last decades?"
The AKP insists it has no Islamic agenda, but its leaders are former Islamists, and Turkey's secular establishment - which includes the military - does not believe their views have changed.
It says the government's move to lift a ban on wearing the Islamic headscarf at universities is evidence of its intentions. That reform has already been struck down by the Constitutional Court.
Analysts believe this presages a negative ruling against the AKP when the Constitutional Court rules in several weeks' time.
If that is the case, analysts believe the party may re-form under a different name, and may call fresh elections.
Meanwhile, Turkish media reported that documents seized by police indicated that an illegal ultra-nationalist organisation wanted to foment chaos in Turkey, to provoke the military to intervene and topple the government.
It follows the arrest of several people this week, including two retired generals, after a year-long investigation. Many were known opponents of the government.
They are the most senior generals yet arrested over their alleged links to the Ergenekon group.
The prosecutor is expected to release his indictment giving details of the evidence in that case very soon.
The government insisted that the arrests had nothing to do with the court case against the AKP.
But critics suggested the case was being used to suppress government opponents.
"There is a suspicion in society that it is turning out to be a political revenge process rather than a legal process," said Turkey's main opposition leader Deniz Baykal.