The ruling pro-reform, Islamist-rooted Justice and Development Party (AKP) is seen as likely to win a fresh mandate, but strong gains by nationalist and secularist opposition parties could slash its majority and result in slower reforms.
Polls opened at 7 a.m. in eastern Turkey, while the voting in the rest of the country began one hour later.
Some 42 million people out of a population of 74 million are eligible to vote in the poll. Participation is traditionally high, and media have predicted that more than 80 percent of voters could cast their ballots.
Reports say many people have postponed or cut short holidays in order to go home to vote.
Several voters in Ankara spoke to Radio Farda ahead of today's vote. "I will give my vote to those parties who emphasize on improving life of workers, civil servants and retirees," said Arzu, a 23-year-old woman. "Those who realize the importance of education and the health system, and defend [founder of the Turkish Republic] Mustafa Kemal Ataturk's ideas and principles."
Reza Karaman, 40, said he has other priorities. "I will vote for a leader who fights against terrorist activities," he said.
Today's election was called early by Prime Minister Recep Tayyip Erdogan to defuse a political crisis over the Islamist-oriented ruling party's choice of presidential candidate.
The county's powerful military and secular parties had blocked the nomination of Foreign Minister Abdullah Gul, whose wife wears a head scarf.
They said that Turkey's secularism was in danger. However, the claim was dismissed by the ruling AKP, which also denies it wants to turn Turkey into an Iranian style theocracy.
The "Milliyet" newspaper on July 21 quoted Gul as saying that the military's warning had helped his party during the campaign because voters were angry at the military's effort to influence the political process.
Some analysts view this election as one of the most important in lthe ast 25 years, because it is seen as a key to Turkey's future direction.
Voter surveys suggest the ruling AKP will retain a majority in the 550-member parliament.
Erdogan, Turkey's most popular politician, on July 21 urged voters to grant him a fresh five-year mandate to continue the AKP's record of strong economic growth, rising living standards, and falling inflation. "We are in the final hours," Erdogan said. "God willing, after 30 hours, Turkey is going to be brighter with AK party's bulb [the symbol of AKP]."
Only two other parties -- the center-left nationalist Republican People's Party (CHP) and far-right Nationalist Movement Party (MHP) -- look set to pass the high 10 percent threshold to enter parliament.
They say the vote is about defending Turkey's secular system.
Some independent, mostly pro-Kurdish candidates are also tipped to win seats in the parliament.
Polling stations close at 4 p.m. local time in eastern Turkey and an hour later in western Turkey. Unofficial results are due this evening.
The new parliament will be immediately faced with
several issues, including a presidential election and a continuing conflict with
Kurdish separatist rebels, some based in neighboring Iraq.
(Radio Farda, with material from news agencies)