A billboard carrying a picture of Turkish President Tayyip Erdogan and a slogan reading, "People say Yes, the Republic gets stronger", is pictured ahead of the constitutional referendum in Istanbul, Turkey, April 11, 2017. REUTERS/Murad Sezer

ISTANBUL (Reuters) - Facing stiff competition from rivals in Istanbul, taxi hailing app Uber will offer free rides in the Turkish city on Sunday for voters taking part in the hotly contested referendum.

Turks will vote on whether to grant President Tayyip Erdogan sweeping powers and bring about one of the biggest changes in the country’s system of governance since the modern republic was founded almost a century ago.

“Uber will take everyone to the ballot boxes for free in the referendum being held on April 16,” the company said in a statement.

The offer only applies in Istanbul -- a city of 15 million people -- and is limited to two journeys worth up to 30 Turkish lira ($8.07) each on the uberXL service, referring to larger vehicles that hold up to six people.

Uber launched in Istanbul in 2014 and has faced competition from a popular home-grown taxi app, Bitaksi. It has previously offered a boat service aiming to capitalize on Istanbul’s infamous road congestion.

Polls show a close race in the referendum, with a slight lead for a “yes” vote to support changes that would replace Turkey’s parliamentary system with the executive presidency Erdogan says is needed to give the country strength.

It would enable the president to issue decrees, declare emergency rule, appoint ministers and top state officials, and could see Erdogan remain in power in the NATO member state until 2029.

Opponents see it as another step toward one-man rule that would likely erode basic rights and freedoms.