Turkey's Constitutional Court is meeting to consider if the governing AK Party should be banned for alleged anti-secular activities.
It is the culmination of a series of clashes between the party, which has Islamist roots, and the secular elite.
The AKP, which won a huge poll victory last year, denies it wants to create an Islamist state by stealth, and calls the case an attack on democracy.
Hours before the court opened, bomb attacks in Istanbul killed 16 people.
More than 150 people were wounded in the twin bombings.
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'Judicial coup'
The case represents a clash of powerful forces.
The modern Turkish republic was founded in 1923 as a secular and unitary state.
Since the 1960s, more than 20 parties - mostly pro-Islamist or pro-Kurdish - have been shut down by the courts for allegedly posing a threat to those principles.
However, this is the first time that a closure case has been brought against a governing party with a huge parliamentary majority, the BBC's Pam O'Toole says.
The stakes are high, our correspondent says, with the AKP fighting for its political survival, and the secularists viewing the case as their last opportunity to block what they allege are attempts to turn Turkey into an Islamic state.
AKP members, including President Abdullah Gul and Prime Minister Recep Tayyip Erdogan, could be banned from political activities for five years.
The party argues that it is facing a "judicial coup".
The Constitutional Court has said it will convene on a daily basis until the 11 judge-panel reaches one of three verdicts - to shut down the party and impose political bans on its members; to cut treasury aid to it; or to throw out the case.
If the party is closed or large numbers of its senior members are banned from politics, it will deepen the political fault lines between AKP supporters and secularists, our correspondent says.
This could lead to a period of political instability or even another general election, with the AKP's deputies regrouping under a different name, she adds.
Last week, an adviser to the court recommended that it should not shut down the AKP, arguing its decision to lift a ban on Islamic headscarves in universities had not challenged the constitution.
The case has already caused uncertainty in Turkey's stock market.
There is also speculation that a ban could harm Ankara's long-running bid to join the European Union, which has expressed concern.
During its first term in office, the AKP pushed through major reforms aimed at EU membership.
But critics allege that in its second term, it has focused more on policies aimed at its conservative supporters.
(BBC)
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