Turks are voting in a general election which will determine whether the ruling party can change the constitution.
President Recep Tayyip Erdogan who first came to power as prime minister in 2003, is seeking a big enough majority to turn Turkey into a presidential republic.
However his hopes may be scuppered if the pro-Kurdish HDP crosses the 10% threshold and enters parliament.
Explosions at its election rally in Diyarbakir on Friday killed four.
Officials said the blasts were caused by improvised bombs.
If the left-wing HDP succeeds in winning seats in parliament for the first time, it would reduce the number of seats won by Mr Erdogan’s AKP, thwarting its plans to change the constitution and transfer the prime minister’s executive powers to the president.
A strong showing from the main opposition Republican People’s Party (CHP) and the third-placed Nationalist Movement Party (MHP) could even force the AKP into a coalition, correspondents say.
Mr Erdogan served as prime minister until he won the presidential election last year.
The BBC’s Mark Lowen in Istanbul says Sunday’s election is the biggest electoral challenge for the AKP since it came to power 13 years ago.
Growth has stalled, he says, critics talk of an authoritarian President Erdogan who has eroded free speech and burnt bridges with the West – and they are desperate for change.
The result may have ramifications beyond Turkey’s borders.
The country is a vital Nato member in a volatile Middle East and a rare mix of Islam and democracy, our correspondent notes.
This report was originally published by the BBC