Istanbul, Apr 9 (IANS): Turkey's political parties held mass rallies on Saturday, a week ahead of a referendum on whether to shift to the presidential system.

The "yes" campaign led by the ruling Justice and Development Party and President Recep Tayyip Erdogan gathered in Istanbul's Yenikapi Square for its biggest rally so far, with the participation of nearly one million people, Xinhua news agency reported.

"Is Istanbul ready to say 'yes' on April 16?" the president asked the crowd.

Erdogan argued that a "yes" vote will contribute to the Turkish struggle against the Islamic State terrorist group and the Kurdistan Workers' Party (PKK), which is listed as a terror organisation by Turkey, the US and the European Union.

He vowed that Turkey would fight terrorism in a more effective way under the presidential system.

The president also argued that voting "yes" would be the best answer to the country's struggle against Fethullah Gulen, a US-based preacher blamed by the Turkish government for orchestrating a failed coup in July last year.

He called for three million Turkish voters living abroad to cast their votes, saying the upcoming referendum will be a breakthrough in Turkey's history.

Kemal Kilicdaroglu, leader of the main opposition Republican People's Party, appeared at a rally in the northern city of Trabzon.

He reiterated his party's argument that the constitutional amendments would restrict the powers of parliament.

"If you say 'yes,' you will give the whole authorisation to one person," Kilicdaroglu warned.

"So that without any justification, the president will be able to abolish the parliament."

Turkey's pro-Kurdish People's Democratic Party also held a rally in Istanbul, calling for a "no" vote in the plebiscite for the sake of peace.

The party's 13 lawmakers, including its co-chairs, are in prison over their alleged links to PKK.