With about 88 percent of the vote counted, President Recep Tayyip Erdogan's ruling Justice and Development Party, the AKP, was well ahead of other parties with the support of just over 42 percent, according to state-run TRT television. But the projections had it around just 270 seats - the bare minimum to keep its majority.
More than 53 million voters in Turkey and abroad were eligible to choose the deputies to the Grand National Assembly. AKP would need a majority of 330 seats of the total 550 to call for a national referendum to change the constitution. With 367 seats, it would be able vote in a change without a referendum.
In the biggest threat to the ruling party's chances, the main Kurdish party was running at over 11 percent - above the 10 percent minimum threshold for representation in Parliament.
The main secular opposition Republican Peoples Party, or CHP was at about 25 percent of the vote, while the nationalist MHP was just under 17 percent.
AKP received around 49 percent of the vote in general elections in 2011. If the current trend holds, it would be first time that the party is faced with falling short of a majority to rule alone since it swept into power in 2002.
Erdogan himself was not on the ballot. Still, the election was effectively a referendum on whether to endow his office with extraordinary powers that would significantly change Turkey's democracy and prolong his reign as the country's most powerful politician.
HDP's apparent leap above the 10 percent threshold, would vault it into a significant position in the Parliament, winning seats greatly at the cost of the ruling party's current majority and making constitutional change on AKP's terms unlikely.
The HDP seemed to have made considerable gains in southeast Turkey, suggesting that religious Kurds had turned away from AKP in favor of HDP.
AKP also appeared to have lost votes in Sanliurfa and Gaziantep where there are large numbers of Syrian refugees.
The vote came amid high tensions after bombings Friday during a HDP rally killed 2 people and wounded scores. On Sunday, Prime Minister Ahmet Davutoglu said a suspect had been detained in the case, but provided no other details.
After casting his vote, HDP leader Selahattin Demirtas called for peace after what he saw as an "onerous and a troubled campaign."
Aside from the constitutional issues, the election could have a major impact on the peace process to end decades of insurgency by Kurdish militants in Turkey.
Scuffles between rival party supporters were reported in at least two provinces Sunday, including one in Sanliurfa which injured 15 people.
Erdogan has been Turkey's dominant politician since his party swept into power in 2002 -- becoming prime minister in 2003 and leading his party to two overwhelming parliamentary election victories. In a gamble last year, he decided to run for president, banking that his party could later bolster his powers.
Under the current constitution, Erdogan is meant to stay above the political fray as president. But he has been campaigning vociferously, drawing complaints from the opposition that he is ignoring the constitution.
As he cast his vote Sunday, Erdogan praised the election as an indication of the strength of democracy in Turkey.
"This strong democracy will be confirmed with the will of our people and extend the trust we have in our future," Erdogan said.
Early in the campaign, he called on voters to give AKP 400 deputies. His party appears to have fallen well short, a development that could leave Erdogan stranded in the presidential palace without the powers he has long sought.
A narrow win by the AKP, however, could be a good result for Davutoglu, who would lose power if Erdogan has his way.
After the final official results are confirmed there is a 45-day period in which a new government needs to be formed, or new elections are called.