ISTANBUL - As soon as school is over, Burcu and Ece take a bus to Besiktas Square in central Istanbul. The moment they are there, the bosom buddies, both aged 15, pick up "No" flyers from tents set up for a referendum campaign and start passing them out.

"Go to the polls and cast your vote," shout the girls, who are wearing identical flower hair bands. Their shouts are mostly lost in the street noise.

Turks face a historic referendum on Sunday, when they must decide whether to grant the country's president more sweeping powers and curb those of the parliament.