Polling began at 7 a.m. local time (0400GMT) to take earlier daylight hours into account
Voting in 32 eastern provinces of Turkey ended at 4.00 p.m. local time (1300GMT) on Sunday.
Voting in the rest of the country is due to continue to 5.00 p.m. local time (1400GMT).
A referendum is being held on an 18-article constitutional reform bill that would shift the country from a parliamentary to a presidential system.
The 32 provinces are Adiyaman, Agri, Artvin, Bingol, Bitlis, Diyarbakir, Elazig, Erzincan, Erzurum, Gaziantep, Giresun, Gumushane, Hakkari, Kars, Malatya, Kahramanmaras, Mardin, Mus, Ordu, Rize, Siirt, Sivas, Trabzon, Tunceli, Sanliurfa, Van, Bayburt, Batman, Sirnak, Ardahan, Igdir and Kilis.
Voting in the east began an hour earlier at 7.00 a.m. to account for earlier daylight hours.